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Herkimer diamonds are not actually diamonds, but are double-terminated quartz crystals of exceptional clarity (water-clear) discovered within exposed outcrops of dolostone in and around Herkimer County, New York and the Mohawk River Valley. The "diamond" in their name is due to both their clarity and natural faceting - crystals possess double termination points and 18 total facets (six on each point, six around the center). Because the first discovery sites were in the village of Middleville and in the city of Little Falls, respectively, the crystal is also known as a Middleville diamond or a Little Falls diamond.
Herkimer diamonds became widely recognized after workmen discovered them in large quantities while cutting into the Mohawk River Valley dolostone in the late 18th century. Geologists discovered exposed dolostone in Herkimer County outcroppings and began mining there, leading to the "Herkimer diamond" moniker. Double-point 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 Adirondak Mountains to the north. The calcium and magnesium carbonate sediments accumulated and lithified to form the dolostone bedrock exposed as the Little Falls Dolostone today. While buried, cavities were formed by acidic waters forming the vugs in which the quartz crystals formed. While the dolostone unit is Cambrian in age the quartz within the vugs is interpreted to have formed during the Carboniferous Period. Waxy organic material, silicon dioxide and pyrite (iron sulfide) was present as minor constituents of rock made of dolomite and calcite. As sediment buried the rock and temperatures rose, crystals grew in the cavities very slowly, resulting in quartz crystals of exceptional clarity. Inclusions can be found in these crystals that provide clues to the origins of the Herkimer diamonds. Found within the inclusions are solids, liquids (salt water or petroleum), gases (most often carbon dioxide), two- and three-phase inclusions, and negative (uniaxial) crystals. Anthraxolite is the most common solid inclusion.