Herkimer Diamond is the name given to a rare gem variety of quartz, belonging to the rock crystal family of stones. Although similar crystals form all over the world, true Herkimers only come from Herkimer in New York, USA. They are highly valued and prized for their internal ‘diamond like’ brightness. All rock crystals are composed of SiO₂ They form according to a trigonal crystal structure, and have a hardness of 7 on the Mohs scale.