Quartz was chosen by the geologist Friedrich Mohs to represent grade 7 in his famous scale of hardness dating back to 1825, and which is still valid nowadays. The so-called Herkimer diamonds are crystals of hyaline quartz, generally two-headed and they appear in the cavities of a gray rock in Herkimer county, New York (USA). They are called "diamonds" because of their outstanding luster and transparency. On a few occasions, crystals unite to form spectacular groupings which are highly valued and sought after by collectors.