Stamp sand is a coarse sand left over from the processing of ore in a stamp mill.[1] In the United States, the most well-known deposits of stamp sand are in the Copper Country of northern Michigan, where it is black or dark gray, and may contain hazardous concentrations of trace metals.
In the 19th and early 20th centuries, many metal mines used stamp mills to process ore-bearing rock. The rock was brought to a stamp mill to be crushed. After crushing the material was mechanically separated to extract metals, or chemically treated by acids if the metal could be leached out. The size of the crushed material depended on the nature of the ore found in each mining district.
Stamp sand, photographed near Houghton, MI
https://en.wikipedia.org/wiki/Stamp_sand
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