Herkimer diamond pendant on copper necklace. Chain is about 10 inches long. Worn once. From a smoke free and pet free home. Herkimer diamonds are double-terminated quartz crystals discovered within exposed outcrops of dolomite in and around Herkimer County, New York, and the Mohawk River Valley in the US.[1][2] They are not diamonds; the "diamond" in their name is due to both their clarity and well formed faces.