Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each storing one bit of data. The material is arranged to form a series of parallel tracks that the bubbles can move along under the action of an external magnetic field. The bubbles are read by moving them to the edge of the material, where they can be read by a conventional magnetic pickup, and then rewritten on the far edge to keep the memory cycling through the material. In operation, bubble memories are similar to delay-line memory systems.
Bubble memory started out as a promising technology in the 1970s, offering memory density of an order similar to hard drives, but performance more comparable to core memory, while lacking any moving parts. This led many to consider it a contender for a "universal memory" that could be used for all storage needs. The introduction of dramatically faster semiconductor memory chips pushed bubble into the slow end of the scale, and equally dramatic improvements in hard-drive capacity made it uncompetitive in price terms.[1] Bubble memory was used for some time in the 1970s and 1980s where its non-moving nature was desirable for maintenance or shock-proofing reasons. The introduction of flash storage and similar technologies rendered even this niche uncompetitive, and bubble disappeared entirely by the late 1980s.
History
Precursors
Bubble memory is largely the brainchild of a single person, Andrew Bobeck. Bobeck had worked on many kinds of magnetics-related projects through the 1960s, and two of his projects put him in a particularly good position for the development of bubble memory. The first was the development of the first magnetic-core memory system driven by a transistor-based controller, and the second was the development of twistor memory.
Twistor is essentially a version of core memory that replaces the "cores" with a piece of magnetic tape. The main advantage of twistor is its ability to be assembled by automated machines, as opposed to core, which was almost entirely manual. AT&T had great hopes for twistor, believing that it would greatly reduce the cost of computer memory and put them in an industry leading position. Instead, DRAM memories came onto the market in the early 1970s and rapidly replaced all previous random-access memory systems. Twistor ended up being used only in a few applications, many of them AT&T's own computers.
One interesting side effect of the twistor concept was noticed in production: under certain conditions, passing a current through one of the electrical wires running inside the tape would cause the magnetic fields on the tape to move in the direction of the current. If used properly, it allowed the stored bits to be pushed down the tape and pop off the end, forming a type of delay-line memory, but one where the propagation of the fields was under computer control, as opposed to automatically advancing at a set rate defined by the materials used. However, such a system had few advantages over twistor, especially as it did not allow random access.
Development
In 1967, Bobeck joined a team at Bell Labs and started work on improving twistor. The memory density of twistor was a function of the size of the wires; the length of any one wire determined how many bits it held, and many such wires were laid side-by-side to produce a larger memory system.
Conventional magnetic materials, like the magnetic tape used in twistor, allowed the magnetic signal to be placed at any location and to move in any direction. Paul Charles Michaelis working with permalloy magnetic thin films discovered that it was possible to move magnetic signals in orthogonal directions within the film. This seminal work led to a patent application.[2] The memory device and method of propagation were described in a paper presented at the 13th Annual Conference on Magnetism and Magnetic Materials, Boston, Massachusetts, 15 September 1967. The device used anisotropic thin magnetic films that required different magnetic pulse combinations for orthogonal propagation directions. The propagation velocity was also dependent on the hard and easy magnetic axes. This difference suggested that an isotropic magnetic medium would be desirable.
This led to the possibility of making a memory system similar to the moving-domain twistor concept, but using a single block of magnetic material instead of many twistor wires. Starting work extending this concept using orthoferrite, Bobeck noticed an additional interesting effect. With the magnetic tape materials used in twistor, the data had to be stored on relatively large patches known as domains. Attempts to magnetize smaller areas would fail. With orthoferrite, if the patch was written and then a magnetic field was applied to the entire material, the patch would shrink down into a tiny circle, which he called a bubble. These bubbles were much smaller than the domains of normal media like tape, which suggested that very high area densities were possible.
Five significant discoveries took place at Bell Labs:
- The controlled two-dimensional motion of single wall domains in permalloy films
- The application of orthoferrites
- The discovery of the stable cylindrical domain
- The invention of the field access mode of operation
- The discovery of growth-induced uniaxial anisotropy in the garnet system and the realization that garnets would be a practical material
The bubble system cannot be described by any single invention, but in terms of the above discoveries. Andy Bobeck was the sole discoverer of (4) and (5) and co-discoverer of (2) and (3); (1) was performed by P. Michaelis in P. Bonyhard's group. At one point, over 60 scientists were working on the project at Bell Labs, many of whom have earned recognition in this field. For instance, in September 1974, H.E.D. Scovil, P.C. Michaelis and Bobeck were awarded the IEEE Morris N. Liebmann Memorial Award by the IEEE with the following citation: For the concept and development of single-walled magnetic domains (magnetic bubbles), and for recognition of their importance to memory technology.
It took some time to find the perfect material, but it was discovered that some garnets had the correct properties. Bubbles would easily form in the material and could be pushed along it fairly easily. The next problem was to make them move to the proper location where they could be read back out: twistor was a wire and there was only one place to go, but in a 2D sheet things would not be so easy. Unlike the original experiments, the garnet did not constrain the bubbles to move only in one direction, but its bubble properties were too advantageous to ignore.
The solution was to imprint a pattern of tiny magnetic bars onto the surface of the garnet, called propagation elements. When a small magnetic field was applied, they would become magnetized, and the bubbles would "stick" to one end. By then reversing the field they would be attracted to the far end, moving down the surface. Another reversal would pop them off the end of the bar to the next bar in the line, and so on, controlling or guiding the direction of travel of the bubbles. T bars/guides, shaped like the letters, were used in early bubble memory designs, but were later replaced by other shapes such as asymmetrical chevrons.[3] In practice the magnetic field rotates and is provided by a pair of coils, that produce a rotating magnetic field in the X and Z axes, it is this rotating magnetic field that moves the bubbles in the memory.
Amorphous magnetic films were also considered as they had greater potential for improvement of bubble memories vs garnet magnetic films, however the existing experience with garnet films meant that they did not gain a foothold. Garnet films have the same or better magnetic properties than orthoferrite films which were considered less promising by comparison. Garnet materials (as films on top of a substrate) could allow for higher propagation speeds of the bubbles (bubble speed) than orthoferrites. Hard bubbles are slower and more erratic than normal bubbles, a problem that is often overcome by ion-implantation of the garnet magnetic film with neon,[4] and can also be done by coating the garnet magnetic film with permalloy.[5]
A memory device is formed by lining up tiny electromagnets at one end with detectors at the other end. Bubbles written in would be slowly pushed to the other, forming a sheet of twistors lined up beside each other. Attaching the output from the detector back to the electromagnets turns the sheet into a series of loops, which can hold the information as long as needed.[3]
Bubble memory is a non-volatile memory. Even when power was removed, the bubbles remained, just as the patterns do on the surface of a disk drive. Better yet, bubble memory devices needed no moving parts: the field that pushed the bubbles along the surface was generated electrically, whereas media like tape and disk drives required mechanical movement. Finally, because of the small size of the bubbles, the density was in theory much higher than existing magnetic storage devices. The only downside was performance; the bubbles had to cycle to the far end of the sheet before they could be read.
A bubble memory device consists of a case, that houses a PCB with connections to one or more bubble memory chips, which may be translucent. The area around the chips on the PCB is surrounded by two windings made of copper wire or other electrically conductive material, that mostly wrap the area, leaving some space for the PCB to pass through the windings and connect to the chips. The windings are wound in directions opposite to each other, for example one winding has wires oriented along the X axis and the other winding has wires along the Z axis. The windings, in turn, are surrounded by two permanent magnets, one below and another above the windings. This forms an assembly that is housed inside the case which acts as a magnetic shield and forms a magnetic return path for the magnetic field from the magnets. The permanent magnets are critical; they create a static (DC, direct current) magnetic field, used as a bias field that enables the contents of the memory to be retained, in other words they allow bubble memories to be non-volatile. If the magnets are removed, all bubbles will disappear and thus all contents will be deleted. The windings create a rotating magnetic field parallel to the orientation of the bubble memory, at around 100 to 200 khz. This will move or drive the bubbles in the magnetic film in a somewhat circular fashion, guided or restrained by the propagation elements. For example, the rotating magnetic field can force the bubbles to constantly circulate around loops, which may be elongated and are defined by the locations of the guiding elements.[3][6]
To allow the bubbles to move around the bubble chips and to guide them through the chip, the chips have some sort of pattern made of ferromagnetic metal that can include for examplie asymmetrical chevrons.[3] For example, the bubbles can move around the edges of the chevrons. The patterns can be called propagation elements as they allow the bubbles to move or propagate across it. They define pathways for the bubbles to be stored and retrieved for reading and the rotating magnetic field moves the bubbles along these paths. For bubble memory, a material like Gadolinium Gallium Garnet is used as the substrate in the chips.[3] On top of the substrate is a magnetic film (bubble host or bubble film/layer)[5][4] such as a Gadolinium-containing garnet[5] or more often, single crystal substituted yttrium iron garnet[4] which holds the magnetic bubbles, that is grown epitaxially with liquid-phase epitaxy with lead oxide flux as the liquid with yttrium oxide and other oxides, and then the film is doped with ion-implantation of one or several elements, to reduce undesirable characteristics.[5][3] The epitaxy process would be carried out with a platinum crucible and wafer holder.[4] The chevrons and other parts are built on top of the film.[3] The propagation elements, including the chevrons, can be made of a material such as Nickel-Iron permalloy. The materials in bubble memories are chosen mainly for their magnetic properties.[3] Gadolinium Gallium Garnet is used as a substrate because it can support the epitaxial growth of magnetic garnet films, and is nonmagnetic,[4] although some bubble memories used Nickel-Cobalt substrates instead.
The use of propagation elements formed by ion implantation instead of permalloy, was proposed to increase the capacity of bubble memory to 16 Mbit/cm2.[4]
Commercialization
Bobeck's team soon had 1 cm (0.39 in) square memories that stored 4,096 bits, the same as a then-standard plane of core memory. This sparked considerable interest in the industry. Not only could bubble memories replace core but it seemed that they could replace tapes and disks as well. In fact, it seemed that bubble memory would soon be the only form of memory used in the vast majority of applications, with the high-performance market being the only one they could not serve.
The technology was included in experimental devices from Bell Labs in 1974.[7] By the mid-1970s, practically every large electronics company had teams working on bubble memory.[8] Texas Instruments introduced the first commercial product that incorporated bubble memory in 1977, and introduced the first commercially available bubble memory, the TIB 0103 with 92 kilobit capacity.[9][10][11] By the late 1970s several products were on the market, and Intel released their own 1-megabit version, the 7110, in 1979.[12][13][14] By the early 1980s, however, bubble memory technology became a dead end with the introduction of hard disk systems offering higher storage densities, higher access speeds, and lower costs. In 1981 major companies working on the technology closed their bubble memory operations,[15] notably Rockwell, National Semiconductor, Texas Instruments and Plessey, leaving a "big five" group of companies still pursuing "second-generation bubble" by 1984: Intel, Motorola, Hitachi, Sagem and Fujitsu.[16] 4-megabit bubble memories such as the Intel 7114, were introduced in 1983[17][18][19] and 16-megabit bubble memory was developed.[20][21]
Bubble memory found uses in niche markets through the 1980s in systems needing to avoid the higher rates of mechanical failures of disk drives, and in systems operating in high vibration or harsh environments. This application became obsolete too with the development of flash storage, which also brought performance, density, and cost benefits.
One application was Konami's Bubble System arcade video game system, introduced in 1984. It featured interchangeable bubble memory cartridges on a 68000-based board. The Bubble System required a "warm-up" time of about 85 seconds (prompted by a timer on the screen when switched on) before the game was loaded, as bubble memory needs to be heated to around 30 to 40 °C (86 to 104 °F) to operate properly. Fujitsu used bubble memory on their FM-8 in 1981 and Sharp used it in their PC 5000 series, a laptop-like portable computer from 1983. Nicolet used bubble memory modules for saving waveforms in their Model 3091 oscilloscope, as did HP who offered a $1595 bubble memory option that extended the memory on their model 3561A digital signal analyzer. GRiD Systems Corporation used it in their early laptops. TIE communication used it in the early development of digital phone systems in order to lower their MTBF rates and produce a non-volatile telephone system's central processor.[22] Bubble memory was also used on the Quantel Mirage DVM8000/1 VFX system.[citation needed]
To store the bubbles, the propagation elements are in pairs and side to side, and are arranged in rows called loops to store the bubbles, thus they are storage loops since the bubbles that are stored in a loop will constantly circulate around it, forced by the rotating magnetic field that can also move the bubbles elsewhere. Bubble memories have extra spare loops to allow for increased yield during manufacturing as they replace defective loops. The list of defective loops is programmed onto the memory, on a special, separate loop called a boot loop, and it is also often printed on the label of the memory. A bubble memory controller will read the boot loop every time a bubble memory system is powered on, during initialization the controller will put the boot loop data in a boot loop register. Writing into a bubble memory is done by a formatter within the memory controller and signals from bits read in the bubble memory are amplified by the sense amplifier of the controller and they will reference the boot loop register to avoid overwriting, or further reading of the data in the boot loop.[3]
The bubbles are created (the memory is written) with a seed bubble that is constantly split or cut by a hairpin-shaped piece of electrically conductive wire (such as aluminum-copper alloy) using a current strong enough to locally overcome and reverse the magnetic bias field generated by the magnets, thus the hairpin-shaped piece of wire acts as a small electromagnet. The seed bubble regains its original size quickly after cutting. The seed bubble circulates under a circular permalloy patch which keeps it from moving elsewhere. After generation, the bubbles then circulate into an "input track" and then into a storage loop. Old bubbles would be moved out of the loop into an "output track" for destruction later. The space left behind by the old bubbles would then be available for new ones.[3] If the seed bubble is ever lost, a new one can be nucleated via special signals sent to the bubble memory and a current 2 to 4 times higher than necessary for cutting of bubbles from the seed bubble.[4]
The bubbles in a storage loop (and empty spaces for bubbles) constantly circulate around it. To read a bubble, it would be "replicated" by moving it to a larger propagation element to stretch the bubble, then it would be passed under a hairpin-shaped conductor to cut it into two with a current pulse which lasts 1/4 of a hertz and is shaped as a spike waveform with a long trailing edge, this would split the bubble in two, one of which would continue circulating in the storage loop, keeping the bubble and thus data safe in case of power failure. The other bubble would be moved to an output track to move it to a detector which is a magnetoresistive bridge, made of a column of interconnected permalloy chevrons where the chevrons are one behind the other, and before it there are similar columns of chevrons that are not interconnected. These stretch the bubbles to generate a larger output at the detector. The detector has a constant electric current, and when bubbles pass under it, they change slightly the electrical resistance and thus current in the detector, and the movement of the bubbles creates a voltage in the order of millivolts, and this is read as either a 1 or a 0. Because the bubble must be moved to a specific area to be read, there are latency constraints. After the detector the bubbles are run into a guard rail to destroy them. A 1 is represented by a bubble, and a 0 is represented by the absence of a bubble.[3]
The Gadolinium Gallium Garnet wafers used as substrates for the bubble chips, were 3 inches in diameter and cost $100 each in 1982 as their production required the use of iridium crucibles.[4]
Further applications
In 2007, the idea of using microfluidic bubbles as logic (rather than memory) was proposed by MIT researchers. The bubble logic would use nanotechnology and has been demonstrated to have access times of 7 ms, which is faster than the 10 ms access times that present hard drives have, though it is slower than the access time of traditional RAM and of traditional logic circuits, making the proposal not commercially practical at present.[23]
IBM's 2008 work on racetrack memory is essentially a 1-dimensional version of bubble, bearing an even closer relationship to the original serial twistor concept.[24]
See also
- Gadolinium gallium garnet, used in many bubble memories as a substrate
References
Among manufacturers of magnetic bubble units, besides Bell Labs and I.B.M., are Texas Instruments, the Honeywell Inc. process control division in Phoenix, and Rockwell International...
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- Parkin (11 April 2008). "Magnetic Domain-Wall Racetrack Memory". Science. 320 (5873): 190–4. Bibcode:2008Sci...320..190P. doi:10.1126/science.1145799. PMID 18403702. S2CID 19285283.
External links
- Great Microprocessors of the Past and Present. Appendix F: Memory Types: Web site by John Bayko
- The Arcade Flyer Archive: Konami Bubble System Flyer
- Bubbles: the better memory
- Whatever Happened to Bubble Memory?
- Magnetic Bubble Memories - Web site by George S. Almasi
- Novel Non-magnetic Bubble Memory
- Structure of a bubble memory
- An exploded view and photo of a dissasembled bubble memory, showing PCBs with memory bubble chips
- A file operating system ported to a modern bubble board
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https://en.wikipedia.org/wiki/Bubble_memory
Gadolinium Gallium Garnet (GGG, Gd3Ga5O12) is a synthetic crystalline material of the garnet group, with good mechanical, thermal, and optical properties. It is typically colorless. It has a cubic lattice, a density of 7.08 g/cm3 and its Mohs hardness is variously noted as 6.5 and 7.5. Its crystals are produced with the Czochralski method. During production, various dopants can be added for colour modification. The material is also used in fabrication of various optical components and as a substrate material for magneto–optical films (magnetic bubble memory).[2] It also finds use in jewelry as a diamond simulant. GGG can also be used as a seed substrate for the growth of other garnets such as yttrium iron garnet.[3]
See also
https://en.wikipedia.org/wiki/Gadolinium_gallium_garnet
A diamond simulant, diamond imitation or imitation diamond is an object or material with gemological characteristics similar to those of a diamond. Simulants are distinct from synthetic diamonds, which are actual diamonds exhibiting the same material properties as natural diamonds. Enhanced diamonds are also excluded from this definition. A diamond simulant may be artificial, natural, or in some cases a combination thereof. While their material properties depart markedly from those of diamond, simulants have certain desired characteristics—such as dispersion and hardness—which lend themselves to imitation. Trained gemologists with appropriate equipment are able to distinguish natural and synthetic diamonds from all diamond simulants, primarily by visual inspection.
The most common diamond simulants are high-leaded glass (i.e., rhinestones) and cubic zirconia (CZ), both artificial materials. A number of other artificial materials, such as strontium titanate and synthetic rutile have been developed since the mid-1950s, but these are no longer in common use. Introduced at the end of the 20th century, the lab-grown product moissanite has gained popularity as an alternative to diamond. The high price of gem-grade diamonds, as well as significant ethical concerns of the diamond trade,[1] have created a large demand for diamond simulants.[2]
Desired and differential properties
In order to be considered for use as a diamond simulant, a material must possess certain diamond-like properties. The most advanced artificial simulants have properties which closely approach diamond, but all simulants have one or more features that clearly and (for those familiar with diamond) easily differentiate them from diamond. To a gemologist, the most important of differential properties are those that foster non-destructive testing; most of these are visual in nature. Non-destructive testing is preferred because most suspected diamonds are already cut into gemstones and set in jewelry, and if a destructive test (which mostly relies on the relative fragility and softness of non-diamonds) fails, it may damage the simulant—an unacceptable outcome for most jewelry owners, as even if a stone is not a diamond, it may still be of value.
Following are some of the properties by which diamond and its simulants can be compared and contrasted.
Durability and density
The Mohs scale of mineral hardness is a non-linear scale of common minerals' resistances to scratching. Diamond is at the top of this scale (hardness 10), as it is one of the hardest naturally occurring materials known. (Some artificial substances, such as aggregated diamond nanorods, are harder.) Since a diamond is unlikely to encounter substances that can scratch it, other than another diamond, diamond gemstones are typically free of scratches. Diamond's hardness also is visually evident (under the microscope or loupe) by its highly lustrous facets (described as adamantine) which are perfectly flat, and by its crisp, sharp facet edges. For a diamond simulant to be effective, it must be very hard relative to most gems. Most simulants fall far short of diamond's hardness, so they can be separated from diamond by their external flaws and poor polish.
In the recent past, the so-called "window pane test" was commonly thought to be an assured method of identifying diamond. It is a potentially destructive test wherein a suspect diamond gemstone is scraped against a pane of glass, with a positive result being a scratch on the glass and none on the gemstone. The use of hardness points and scratch plates made of corundum (hardness 9) are also used in place of glass. Hardness tests are inadvisable for three reasons: glass is fairly soft (typically 6 or below) and can be scratched by a large number of materials (including many simulants); diamond has four directions of perfect and easy cleavage (planes of structural weakness along which the diamond could split) which could be triggered by the testing process; and many diamond-like gemstones (including older simulants) are valuable in their own right.
The specific gravity (SG) or density of a gem diamond is fairly constant at 3.52. Most simulants are far above or slightly below this value, which can make them easy to identify if unset. High-density liquids such as diiodomethane can be used for this purpose, but these liquids are all highly toxic and therefore are usually avoided. A more practical method is to compare the expected size and weight of a suspect diamond to its measured parameters: for example, a cubic zirconia (SG 5.6–6) will be 1.7 times the expected weight of an equivalently sized diamond.
Optics and color
Diamonds are usually cut into brilliants to bring out their brilliance (the amount of light reflected back to the viewer) and fire (the degree to which colorful prismatic flashes are seen). Both properties are strongly affected by the cut of the stone, but they are a function of diamond's high refractive index (RI—the degree to which incident light is bent upon entering the stone) of 2.417 (as measured by sodium light, 589.3 nm) and high dispersion (the degree to which white light is split into its spectral colors as it passes through the stone) of 0.044, as measured by the sodium B and G line interval. Thus, if a diamond simulant's RI and dispersion are too low, it will appear comparatively dull or "lifeless"; if the RI and dispersion are too high, the effect will be considered unreal or even tacky. Very few simulants have closely approximating RI and dispersion, and even the close simulants can be separated by an experienced observer. Direct measurements of RI and dispersion are impractical (a standard gemological refractometer has an upper limit of about RI 1.81), but several companies have devised reflectivity meters to gauge a material's RI indirectly by measuring how well it reflects an infrared beam.
Perhaps equally as important is optic character. Diamond and other cubic (and also amorphous) materials are isotropic, meaning that light entering a stone behaves the same way regardless of direction. Conversely, most minerals are anisotropic, which produces birefringence, or double refraction of light entering the material in all directions other than an optic axis (a direction of single refraction in a doubly refractive material). Under low magnification, this birefringence is usually detectable as a visual doubling of a cut gemstone's rear facets or internal flaws. An effective diamond simulant should therefore be isotropic.
Under longwave (365 nm) ultraviolet light, diamond may fluoresce a blue, yellow, green, mauve, or red of varying intensity. The most common fluorescence is blue, and such stones may also phosphoresce yellow—this is thought to be a unique combination among gemstones. There is usually little if any response to shortwave ultraviolet, in contrast to many diamond simulants. Similarly, because most diamond simulants are artificial, they tend to have uniform properties: in a multi-stone diamond ring, one would expect the individual diamonds to fluoresce differently (in different colors and intensities, with some likely to be inert). If all the stones fluoresce in an identical manner, they are unlikely to be diamond.
Most "colorless" diamonds are actually tinted yellow or brown to some degree, whereas some artificial simulants are completely colorless—the equivalent of a perfect "D" in diamond color terminology. This "too good to be true" factor is important to consider; colored diamond simulants meant to imitate fancy diamonds are more difficult to spot in this regard, but the simulants' colors rarely approximate. In most diamonds (even colorless ones) a characteristic absorption spectrum can be seen (by a direct-vision spectroscope), consisting of a fine line at 415 nm. The dopants used to impart color in artificial simulants may be detectable as a complex rare-earth absorption spectrum, which is never seen in diamond.
Also present in most diamonds are certain internal and external flaws or inclusions, the most common of which are fractures and solid foreign crystals. Artificial simulants are usually internally flawless, and any flaws that are present are characteristic of the manufacturing process. The inclusions seen in natural simulants will often be unlike those ever seen in diamond, most notably liquid "feather" inclusions. The diamond cutting process will often leave portions of the original crystal's surface intact. These are termed naturals and are usually on the girdle of the stone; they take the form of triangular, rectangular, or square pits (etch marks) and are seen only in diamond.
Thermal and electrical
Diamond is an extremely effective thermal conductor and usually an electrical insulator. The former property is widely exploited in the use of an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. As most simulants are thermal insulators, the thermistor's heat will not be conducted. This test takes about 2–3 seconds. The only possible exception is moissanite, which has a thermal conductivity similar to diamond: older probes can be fooled by moissanite, but newer thermal and electrical conductivity testers are sophisticated enough to differentiate the two materials. The latest development is nano diamond coating, an extremely thin layer of diamond material. If not tested properly it may show the same characteristics as a diamond.
A diamond's electrical conductance is only relevant to blue or gray-blue stones, because the interstitial boron responsible for their color also makes them semiconductors. Thus, a suspected blue diamond can be affirmed if it completes an electric circuit successfully.
Artificial simulants
Diamond has been imitated by artificial materials for hundreds of years; advances in technology have seen the development of increasingly better simulants with properties ever nearer those of diamond. Although most of these simulants were characteristic of a certain time period, their large production volumes ensured that all continue to be encountered with varying frequency in jewelry of the present. Nearly all were first conceived for intended use in high technology, such as active laser mediums, varistors, and bubble memory. Due to their limited present supply, collectors may pay a premium for the older types.
Summary table
| Material | Formula | Refractive index(es) 589.3 nm |
Dispersion 431–687 nm |
Hardness (Mohs' scale) |
Density (g/cm3) |
Thermal condc. |
State of the art |
|---|---|---|---|---|---|---|---|
| Diamond | C | 2.417 | 0.044 | 10 | 3.52 | Excellent |
|
|
|
Artificial simulants | ||||||
| Flint glass | Silica with Pb, Al, Tl | ~ 1.6 | > 0.020 | < 6 | 2.4–4.2 | Poor | 1700– |
| White sapphire | Al2O3 | 1.762–1.770 | 0.018 | 9 | 3.97 | Poor | 1900–1947 |
| Spinel | MgO·Al2O3 | 1.727 | 0.020 | 8 | ~ 3.6 | Poor | 1920–1947 |
| Rutile | TiO2 | 2.62–2.9 | 0.33 | ~ 6 | 4.25 | Poor | 1947–1955 |
| Strontium titanate | SrTiO3 | 2.41 | 0.19 | 5.5 | 5.13 | Poor | 1955–1970 |
| YAG | Y3Al5O12 | 1.83 | 0.028 | 8.25 | 4.55–4.65 | Poor | 1970–1975 |
| GGG | Gd3Ga5O12 | 1.97 | 0.045 | 7 | 7.02 | Poor | 1973–1975 |
| Cubic zirconia | ZrO2 (+ rare earths) | ~ 2.2 | ~ 0.06 | ~ 8.3 | ~ 5.7 | Poor | 1976– |
| Moissanite | SiC | 2.648–2.691 | 0.104 | 8.5–9.25 | 3.2 | High | 1998– |
|
|
Natural simulants | ||||||
| Quartz | Silica | 1.543–1.554 |
|
7 | 2.50–2.65 |
|
Ancient |
| Andradite | Ca3Fe2(SiO4)3 | 1.61–1.64 | 0.057 | 6.5–7 | 3.8–3.9 | Poor | Ancient |
| Zircon | ZrSiO4 | 1.78–1.99 | 0.039 | 6.5–7.5 | 4.6–4.7 | Poor | Ancient |
| Topaz | Al2SiO4(F, OH)2 | 1.61–1.64 | 0.014 | 8 | 3.4–3.6 | Poor | Ancient |
The "refractive index(es)" column shows one refractive index for singly refractive substances, and a range for doubly refractive substances.
1700 onwards
The formulation of flint glass using lead, alumina, and thallium to increase RI and dispersion began in the late Baroque period. Flint glass is fashioned into brilliants, and when freshly cut they can be surprisingly effective diamond simulants. Known as rhinestones, pastes, or strass, glass simulants are a common feature of antique jewelry; in such cases, rhinestones can be valuable historical artifacts in their own right. The great softness (below hardness 6) imparted by the lead means a rhinestone's facet edges and faces will quickly become rounded and scratched. Together with conchoidal fractures, and air bubbles or flow lines within the stone, these features make glass imitations easy to spot under only moderate magnification. In contemporary production it is more common for glass to be molded rather than cut into shape: in these stones the facets will be concave and facet edges rounded, and mold marks or seams may also be present. Glass has also been combined with other materials to produce composites.
1900–1947
The first crystalline artificial diamond simulants were synthetic white sapphire (Al2O3, pure corundum) and spinel (MgO·Al2O3, pure magnesium aluminium oxide). Both have been synthesized in large quantities since the first decade of the 20th century via the Verneuil or flame-fusion process, although spinel was not in wide use until the 1920s. The Verneuil process involves an inverted oxyhydrogen blowpipe, with purified feed powder mixed with oxygen that is carefully fed through the blowpipe. The feed powder falls through the oxy-hydrogen flame, melts, and lands on a rotating and slowly descending pedestal below. The height of the pedestal is constantly adjusted to keep its top at the optimal position below the flame, and over a number of hours the molten powder cools and crystallizes to form a single pedunculated pear or boule crystal. The process is an economical one, with crystals of up to 9 centimeters (3.5 inches) in diameter grown. Boules grown via the modern Czochralski process may weigh several kilograms.
Synthetic sapphire and spinel are durable materials (hardness 9 and 8) that take a good polish; however, due to their much lower RI when compared to diamond (1.762–1.770 for sapphire, 1.727 for spinel), they are "lifeless" when cut. (Synthetic sapphire is also anisotropic, making it even easier to spot.) Their low RIs also mean a much lower dispersion (0.018 and 0.020), so even when cut into brilliants they lack the fire of diamond. Nevertheless, synthetic spinel and sapphire were popular diamond simulants from the 1920s until the late 1940s, when newer and better simulants began to appear. Both have also been combined with other materials to create composites. Commercial names once used for synthetic sapphire include Diamondette, Diamondite, Jourado Diamond', and Thrilliant. Names for synthetic spinel included Corundolite, Lustergem, Magalux, and Radiant.
1947–1970
The first of the optically "improved" simulants was synthetic rutile (TiO2, pure titanium oxide). Introduced in 1947–48, synthetic rutile possesses plenty of life when cut—perhaps too much life for a diamond simulant. Synthetic rutile's RI and dispersion (2.8 and 0.33) are so much higher than diamond that the resultant brilliants look almost opal-like in their display of prismatic colors. Synthetic rutile is also doubly refractive: although some stones are cut with the table perpendicular to the optic axis to hide this property, merely tilting the stone will reveal the doubled back facets.
The continued success of synthetic rutile was also hampered by the material's inescapable yellow tint, which producers were never able to remedy. However, synthetic rutile in a range of different colors, including blues and reds, were produced using various metal oxide dopants. These and the near-white stones were extremely popular if unreal stones. Synthetic rutile is also fairly soft (hardness ~6) and brittle, and therefore wears poorly. It is synthesized via a modification of the Verneuil process, which uses a third oxygen pipe to create a tricone burner; this is necessary to produce a single crystal, due to the much higher oxygen losses involved in the oxidation of titanium. The technique was invented by Charles H. Moore, Jr. at the South Amboy, New Jersey-based National Lead Company (later NL Industries). National Lead and Union Carbide were the primary producers of synthetic rutile, and peak annual production reached 750,000 carats (150 kg). Some of the many commercial names applied to synthetic rutile include: Astryl, Diamothyst, Gava or Java Gem, Meredith, Miridis, Rainbow Diamond, Rainbow Magic Diamond, Rutania, Titangem, Titania, and Ultamite.
National Lead was also where research into the synthesis of another titanium compound—strontium titanate (SrTiO3, pure tausonite)—was conducted. Research was done during the late 1940s and early 1950s by Leon Merker and Langtry E. Lynd, who also used a tricone modification of the Verneuil process. Upon its commercial introduction in 1955, strontium titanate quickly replaced synthetic rutile as the most popular diamond simulant. This was due not only to strontium titanate's novelty, but to its superior optics: its RI (2.41) is very close to that of diamond, while its dispersion (0.19), although also very high, was a significant improvement over synthetic rutile's psychedelic display. Dopants were also used to give synthetic titanate a variety of colors, including yellow, orange to red, blue, and black. The material is also isotropic like diamond, meaning there is no distracting doubling of facets as seen in synthetic rutile.
Strontium titanate's only major drawback (if one excludes excess fire) is fragility. It is both softer (hardness 5.5) and more brittle than synthetic rutile—for this reason, strontium titanate was also combined with more durable materials to create composites. It was otherwise the best simulant around at the time, and at its peak annual production was 1.5 million carats (300 kg). Due to patent coverage, all US production was by National Lead, while large amounts were produced overseas by Nakazumi Company of Japan. Commercial names for strontium titanate included Brilliante, Diagem, Diamontina, Fabulite, and Marvelite.
1970–1976
From about 1970 strontium titanate began to be replaced by a new class of diamond imitations: the "synthetic garnets". These are not true garnets in the usual sense because they are oxides rather than silicates, but they do share natural garnet's crystal structure (both are cubic and therefore isotropic) and the general formula A3B2C3O12. While in natural garnets C is always silicon, and A and B may be one of several common elements, most synthetic garnets are composed of uncommon rare-earth elements. They are the only diamond simulants (aside from rhinestones) with no known natural counterparts: gemologically they are best termed artificial rather than synthetic, because the latter term is reserved for human-made materials that can also be found in nature.
Although a number of artificial garnets were successfully grown, only two became important as diamond simulants. The first was yttrium aluminium garnet (YAG; Y3Al5O12) in the late 1960s. It was (and still is) produced by the Czochralski, or crystal-pulling, process, which involves growth from the melt. An iridium crucible surrounded by an inert atmosphere is used, wherein yttrium oxide and aluminium oxide are melted and mixed together at a carefully controlled temperature near 1980 °C. A small seed crystal is attached to a rod, which is lowered over the crucible until the crystal contacts the surface of the melted mixture. The seed crystal acts as a site of nucleation; the temperature is kept steady at a point where the surface of the mixture is just below the melting point. The rod is slowly and continuously rotated and retracted, and the pulled mixture crystallizes as it exits the crucible, forming a single crystal in the form of a cylindrical boule. The crystal's purity is extremely high, and it typically measures 5 cm (2 inches) in diameter and 20 cm (8 inches) in length, and weighs 9,000 carats (1.75 kg).
YAG hardness (8.25) and lack of brittleness were great improvements over strontium titanate, and although its RI (1.83) and dispersion (0.028) were fairly low, they were enough to give brilliant-cut YAGs perceptible fire and good brilliance (although still much lower than diamond). A number of different colors were also produced with the addition of dopants, including yellow, red, and a vivid green, which was used to imitate emerald. Major producers included Shelby Gem Factory of Michigan, Litton Systems, Allied Chemical, Raytheon, and Union Carbide; annual global production peaked at 40 million carats (8000 kg) in 1972, but fell sharply thereafter. Commercial names for YAG included Diamonair, Diamonique, Gemonair, Replique, and Triamond.
While market saturation was one reason for the fall in YAG production levels, another was the recent introduction of the other artificial garnet important as a diamond simulant, gadolinium gallium garnet (GGG; Gd3Ga5O12). Produced in much the same manner as YAG (but with a lower melting point of 1750 °C), GGG had an RI (1.97) close to, and a dispersion (0.045) nearly identical to diamond. GGG was also hard enough (hardness 7) and tough enough to be an effective gemstone, but its ingredients were also much more expensive than YAG's. Equally hindering was GGG's tendency to turn dark brown upon exposure to sunlight or other ultraviolet source: this was due to the fact that most GGG gems were fashioned from impure material that was rejected for technological use. The SG of GGG (7.02) is also the highest of all diamond simulants and amongst the highest of all gemstones, which makes loose GGG gems easy to spot by comparing their dimensions with their expected and actual weights. Relative to its predecessors, GGG was never produced in significant quantities; it became more or less unheard of by the close of the 1970s. Commercial names for GGG included Diamonique II and Galliant.
1976 to present
Cubic zirconia or CZ (ZrO2; zirconium dioxide—not to be confused with zircon, a zirconium silicate) quickly dominated the diamond simulant market following its introduction in 1976, and it remains the most gemologically and economically important simulant. CZ had been synthesized since 1930 but only in ceramic form: the growth of single-crystal CZ would require an approach radically different from those used for previous simulants due to zirconia's extremely high melting point (2750 °C), unsustainable by any crucible. The solution found involved a network of water-filled copper pipes and radio-frequency induction heating coils; the latter to heat the zirconia feed powder, and the former to cool the exterior and maintain a retaining "skin" under 1 millimeter thick. CZ was thus grown in a crucible of itself, a technique called cold crucible (in reference to the cooling pipes) or skull crucible (in reference to either the shape of the crucible or of the crystals grown).
At standard pressure zirconium oxide would normally crystallize in the monoclinic rather than cubic crystal system: for cubic crystals to grow, a stabilizer must be used. This is usually Yttrium(III) oxide or calcium oxide. The skull crucible technique was first developed in 1960s France, but was perfected in the early 1970s by Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow. By 1980 annual global production had reached 50 million carats (10,000 kg).
The hardness (8–8.5), RI (2.15–2.18, isotropic), dispersion (0.058–0.066), and low material cost make CZ the most popular simulant of diamond. Its optical and physical constants are however variable, owing to the different stabilizers used by different producers. There are many formulations of stabilized cubic zirconia. These variations change the physical and optical properties markedly. While the visual likeness of CZ is close enough to diamond to fool most who do not handle diamond regularly, CZ will usually give certain clues. For example: it is somewhat brittle and is soft enough to possess scratches after normal use in jewelry; it is usually internally flawless and completely colorless (whereas most diamonds have some internal imperfections and a yellow tint); its SG (5.6–6) is high; and its reaction under ultraviolet light is a distinctive beige. Most jewelers will use a thermal probe to test all suspected CZs, a test which relies on diamond's superlative thermal conductivity (CZ, like almost all other diamond simulants, is a thermal insulator). CZ is made in a number of different colors meant to imitate fancy diamonds (e.g., yellow to golden brown, orange, red to pink, green, and opaque black), but most of these do not approximate the real thing. Cubic zirconia can be coated with diamond-like carbon to improve its durability, but will still be detected as CZ by a thermal probe.
CZ had virtually no competition until the 1998 introduction of moissanite (SiC; silicon carbide). Moissanite is superior to cubic zirconia in two ways: its hardness (8.5–9.25) and low SG (3.2). The former property results in facets that are sometimes as crisp as a diamond's, while the latter property makes simulated moissanite somewhat harder to spot when unset (although still disparate enough to detect). However, unlike diamond and cubic zirconia, moissanite is strongly birefringent. This manifests as the same "drunken vision" effect seen in synthetic rutile, although to a lesser degree. All moissanite is cut with the table perpendicular to the optic axis in order to hide this property from above, but when viewed under magnification at only a slight tilt the doubling of facets (and any inclusions) is readily apparent.
The inclusions seen in moissanite are also characteristic: most will have fine, white, subparallel growth tubes or needles oriented perpendicular to the stone's table. It is conceivable that these growth tubes could be mistaken for laser drill holes that are sometimes seen in diamond (see diamond enhancement), but the tubes will be noticeably doubled in moissanite due to its birefringence. Like synthetic rutile, current moissanite production is also plagued by an as yet inescapable tint, which is usually a brownish green. A limited range of fancy colors have been produced as well, the two most common being blue and green.
Natural simulants
Natural minerals that (when cut) optically resemble white diamonds are rare, because the trace impurities usually present in natural minerals tend to impart color. The earliest simulants of diamond were colorless quartz (A form of silica, which also form obsidian, glass and sand), rock crystal (a type of quartz), topaz, and beryl (goshenite); they are all common minerals with above-average hardness (7–8), but all have low RIs and correspondingly low dispersions. Well-formed quartz crystals are sometimes offered as "diamonds", a popular example being the so-called "Herkimer diamonds" mined in Herkimer County, New York. Topaz's SG (3.50–3.57) also falls within the range of diamond.
From a historical perspective, the most notable natural simulant of diamond is zircon. It is also fairly hard (7.5), but more importantly shows perceptible fire when cut, due to its high dispersion of 0.039. Colorless zircon has been mined in Sri Lanka for over 2,000 years; prior to the advent of modern mineralogy, colorless zircon was thought to be an inferior form of diamond. It was called "Matara diamond" after its source location. It is still encountered as a diamond simulant, but differentiation is easy due to zircon's anisotropy and strong birefringence (0.059). It is also notoriously brittle and often shows wear on the girdle and facet edges.
Much less common than colorless zircon is colorless scheelite. Its dispersion (0.026) is also high enough to mimic diamond, but although it is highly lustrous its hardness is much too low (4.5–5.5) to maintain a good polish. It is also anisotropic and fairly dense (SG 5.9–6.1). Synthetic scheelite produced via the Czochralski process is available, but it has never been widely used as a diamond simulant. Due to the scarcity of natural gem-quality scheelite, synthetic scheelite is much more likely to simulate it than diamond. A similar case is the orthorhombic carbonate cerussite, which is so fragile (very brittle with four directions of good cleavage) and soft (hardness 3.5) that it is never seen set in jewelry, and only occasionally seen in gem collections because it is so difficult to cut. Cerussite gems have an adamantine luster, high RI (1.804–2.078), and high dispersion (0.051), making them attractive and valued collector's pieces. Aside from softness, they are easily distinguished by cerussite's high density (SG 6.51) and anisotropy with extreme birefringence (0.271).
Due to their rarity fancy-colored diamonds are also imitated, and zircon can serve this purpose too. Applying heat treatment to brown zircon can create several bright colors: these are most commonly sky-blue, golden yellow, and red. Blue zircon is very popular, but it is not necessarily color stable; prolonged exposure to ultraviolet light (including the UV component in sunlight) tends to bleach the stone. Heat treatment also imparts greater brittleness to zircon and characteristic inclusions.
Another fragile candidate mineral is sphalerite (zinc blende). Gem-quality material is usually a strong yellow to honey brown, orange, red, or green; its very high RI (2.37) and dispersion (0.156) make for an extremely lustrous and fiery gem, and it is also isotropic. But here again, its low hardness (2.5–4) and perfect dodecahedral cleavage preclude sphalerite's wide use in jewelry. Two calcium-rich members of the garnet group fare much better: these are grossularite (usually brownish orange, rarely colorless, yellow, green, or pink) and andradite. The latter is the rarest and most costly of the garnets, with three of its varieties—topazolite (yellow), melanite (black), and demantoid (green)—sometimes seen in jewelry. Demantoid (literally "diamond-like") especially has been prized as a gemstone since its discovery in the Ural Mountains in 1868; it is a noted feature of antique Russian and Art Nouveau jewelry. Titanite or sphene is also seen in antique jewelry; it is typically some shade of chartreuse and has a luster, RI (1.885–2.050), and dispersion (0.051) high enough to be mistaken for diamond, yet it is anisotropic (a high birefringence of 0.105–0.135) and soft (hardness 5.5).
Discovered the 1960s, the rich green tsavorite variety of grossular is also very popular. Both grossular and andradite are isotropic and have relatively high RIs (around 1.74 and 1.89 respectively) and high dispersions (0.027 and 0.057), with demantoid's exceeding diamond. However, both have a low hardness (6.5–7.5) and invariably possess inclusions atypical for diamond—the byssolite "horsetails" seen in demantoid are one striking example. Furthermore, most are very small, typically under 0.5 carats (100 mg) in weight. Their lusters range from vitreous to subadamantine, to almost metallic in the usually opaque melanite, which has been used to simulate black diamond. Some natural spinel is also deep black and could serve this same purpose.
Composites
Because strontium titanate and glass are too soft to survive use as a ring stone, they have been used in the construction of composite or doublet diamond simulants. The two materials are used for the bottom portion (pavilion) of the stone, and in the case of strontium titanate, a much harder material—usually colorless synthetic spinel or sapphire—is used for the top half (crown). In glass doublets, the top portion is made of almandine garnet; it is usually a very thin slice which does not modify the stone's overall body color. There have even been reports of diamond-on-diamond doublets, where a creative entrepreneur has used two small pieces of rough to create one larger stone.
In strontium titanate and diamond-based doublets, an epoxy is used to adhere the two halves together. The epoxy may fluoresce under UV light, and there may be residue on the stone's exterior. The garnet top of a glass doublet is physically fused to its base, but in it and the other doublet types there are usually flattened air bubbles seen at the junction of the two halves. A join line is also readily visible whose position is variable; it may be above or below the girdle, sometimes at an angle, but rarely along the girdle itself.
The most recent composite simulant involves combining a CZ core with an outer coating of laboratory created amorphous diamond. The concept effectively mimics the structure of a cultured pearl (which combines a core bead with an outer layer of pearl coating), only done for the diamond market.
See also
Footnotes
- "Why Diamond Replicas?". Archived from the original on 2016-10-12. Retrieved 2016-10-11.
References
 | This article has an unclear citation style. (March 2012) |
- Hall, Cally. (1994). Gemstones. p. 63, 70, 121. Eyewitness Handbooks; Kyodo Printing Co., Singapore. ISBN 0-7737-2762-0
- Nassau, Kurt. (1980). Gems Made by Man, pp. 203–241. Gemological Institute of America; Santa Monica, California. ISBN 0-87311-016-1
- O'Donoghue, Michael, and Joyner, Louise. (2003). Identification of Gemstones, pp. 12–19. Butterworth-Heinemann, Great Britain. ISBN 0-7506-5512-7
- Pagel-Theisen, Verena. (2001). Diamond Grading ABC: The Manual (9th ed.), pp. 298–313. Rubin & Son n.v.; Antwerp, Belgium. ISBN 3-9800434-6-0
- Schadt, H. (1996). Goldsmith's Art: 5000 Years of Jewelry and Hollowware, p. 141. Arnoldsche Art Publisher; Stuttgart, New York. ISBN 3-925369-54-6
- Webster, Robert, and Read, Peter G. (Ed.) (2000). Gems: Their Sources, Descriptions and Identification (5th ed.), pp. 65–71. Butterworth-Heinemann, Great Britain. ISBN 0-7506-1674-1
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