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. Currently, Herkimer diamonds can be found in large quantity in at least Herkimer, Fulton, and Montgomery counties, and double-pointed quartz crystals have also been found in abundance in Tibet and Afghanistan, as well as in other countries.
Many of the New York crystals are known for their extreme clarity, and Wiccan and New Age belief systems often ascribe specific occult properties and a wide variety of mystical powers to them.
These quartz crystals, which geologists theorize formed extremely slowly in small solution cavities or vugs, have 18 facets (6 sides) and two terminations. There are also larger cavities that are several feet in diameter that are called "pockets". Herkimer diamonds are found clear, cloudy, smoky or even containing a variety of rare impurities. Impurities (rare and general) can include clusters, scepters, fluid inclusions (sometimes incorrectly called enhydros), phantom, and bridge crystals. A fluid inclusion is a pocket within a crystal containing liquid, usually water, sometimes also methane or oil, and rarely, smaller crystals. A phantom is a crystal containing other visible crystals of the same type, a skeletal crystal contains a series of crystal edge outlines inside the crystal and a hopper crystal has its faces replaced by a step like pattern.