Herkimer diamond

[3] Herkimer diamonds became widely recognized after workmen discovered them in large quantities while cutting into the Mohawk River Valley dolomite in the late 18th century.

Double-terminated quartz crystals may be found in sites around the world, but only those mined in Herkimer County can be given this name.

The geologic history of these crystals began about 500 million years ago in a shallow sea which was receiving sediments from the ancient Adirondack Mountains to the north.

While the dolomite unit is Cambrian in age, the quartz within the vugs is interpreted to have formed during the Carboniferous Period.

[5] Waxy organic material, silicon dioxide and pyrite (iron sulfide) were present as minor constituents of rock made of dolomite and calcite.