Herkimer diamond is the misnomerous name for double-terminated quartz first discovered in exposed outcrops of dolostone at Little Falls in Herkimer County, New York and the Mohawk River Valley. They became largely recognized after workmen discovered them in large quantities while cutting into the Mohawk River Valley dolostone in the late 1700s. 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 lead to the name, Herkimer diamonds. Currently, Herkimer diamonds can be found in large quantity in at least Herkimer, Fulton, and Montgomery counties.

These quartz crystals, which formed extremely slowly in small pockets or vugs, have 18 facets (6 sides) and two terminations. They can be found clear, cloudy, or even containing a variety of rare impurities. Impurities (rare and general) can include clusters, scepters, enhydro, phantom, and bridge crystals. An enhydro contains a water pocket within the crystal, and a phantom is a crystal which contains an image of itself within.

The crystals referred to unofficially as “Herkimer Diamonds” are officially known as “Little Falls Diamonds” because they are found in a rock formation called “Little Falls Dolostone.”
Some years ago a resident of the village of Herkimer bought one of the diamond mines, and started calling the crystals Herkimer Diamonds, although none are found in the village. They are abundant in the City of Little Falls, and the public library there has a large display of the crystals.