The host rock of “Herkimer diamonds” is the Cambrian-age, Little Falls Dolostone. The Little Falls Dolostone was deposited about 500 million years ago, and the “Herkimer diamonds” formed in cavities within the dolostone. These cavities are frequently lined with drusy quartz crystals and are often coated with a tarry hydrocarbon.

Although Herkimer County, New York is the location for which these crystals are named, similar doubly terminated quartz crystals have been found in many other locations, including Arizona, Afghanistan, Norway, Ukraine, and China. They have the same appearance but cannot rightfully be called “Herkimers.” The doubly terminated quartz crystals shown in the accompanying photo are from a deposit in Afghanistan.

You might also hear the names “Little Falls Diamond” or “Middleville Diamond” used in the Herkimer, New York area. These are similar quartz crystals named for communities in Herkimer County near where they were found. The generic name “Herkimer diamonds” is used for specimens from all of these localities.