Herkimer Diamonds are a special form of Quartz that can only be found in Herkimer County, USA. They are usually double terminated, unless broken or attached to matrix and have unusual clarity, occasionally with carbon/water inclusions. Many sellers try to pass off double terminated quartz from Pakistan as Herkimers, but these do not have the same clarity.
Herkimers are the most powerful of all Quartz crystals, and are strong amplifiers of spiritual energy. Excellent for work in meditation and dream work, they are often very small but mighty. Usually, the smaller the crystal, the clearer it is.