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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. They're called "diamonds" not only because of their clarity, but also because when they are found, they look like someone faceted them---but they are naturally "faceted" with 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 largely 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 and began mining there. The popularity of mining for double-terminated quartz in the Herkimer County outcroppings is what led to the name, Herkimer diamonds. Only these crystals found in Herkimer County, New York can be called "Herkimer Diamonds" or "Herkimer Quartz". Other double-pointed quartz crystals have also been found in abundance in Tibet and Afghanistan, as well as in other countries, but these are not Herkimer "diamonds".
The geologic history of these crystals began 495 million years ago in a shallow sea. Waxy organic material along with quartz sand and pyrite was encased in rock made of dolomite and calcite. As sediment buried the rock and temperatures rose, crystals grew 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: 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.
- http://www.mindat.org/min-1877.html Mindat with locations
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