“Herkimer Diamonds” is the name given to the doubly terminated quartz crystals found in Herkimer County, New York and surrounding areas. Examples of these crystals are shown in the following. Note that these crystals have the typical hexagonal habit of quartz, however, instead of having a termination on one end they are doubly terminated. This is a result of the crystals growing with very little or no contact with their host rock. Such doubly terminated crystals are very rare and this is part of what makes Herkimer Diamonds so popular with mineral collectors.
The host rock for 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 often are 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 a few other locations, including Arizona, Afghanistan, Norway, Ukraine and China. They have the same appearance but can not rightfully be called “Herkimers”. The doubly terminated quartz crystals shown in the lower right photo are from a deposit in Afghanistan.
Who Discovered Herkimer Diamonds?
The Herkimer Diamonds of New York are not a recent discovery. The Mohawk Indians and early settlers knew about the crystals. They found them in stream sediments and plowed fields. These people were amazed with the crystals and immediately held them in high esteem.
Herkimer Diamond Mines
Some of the best places to find Herkimer Diamonds today are located along New York State Route 28 in Middleville, New York. (When visiting this area it is important to remember that all land in New York either belongs to the government or is private property. Collecting minerals from government lands is illegal in New York and collecting on private property always requires permission in advance.)
There are two commercial mines on New York State Route 28 at Middleville, New York. These are: Ace of Diamonds Mine and Herkimer Diamond Mine. Both allow collectors to enter and prospect for a nominal fee. Both locations also rent equipment such as hammers, wedges and other small tools. They also have small exhibit areas where you can view and/or purchase specimens.
Mining for Herkimer Diamonds
The key to finding Herkimer Diamonds is a knowledge that they occur in cavities (vugs) within the Little Falls Dolostone. These cavities can be smaller than a pea or several feet across. At both of the mines listed above the Little Falls Dolostone is exposed at the surface and a significant amount of broken rock is scattered across the quarry floor.