SIO2: Quartz is the most common mineral on the earth. It is found in almost every geological environment and also it is at least a component of almost every rock type. It has a hexagonal crystal structure and is made of trigonal crystallized silica. It is most varied in terms of varieties, colors and forms. The most important distinction between the types of quartz is that one is of macro crystalline, which is individual crystal visible to the unaided eye, and the other is micro crystalline or cryptocrystalline varieties, aggregates of crystals visible only under high magnification. Chalcedony is the generic term-for cryptocrystalline quartz. The transparent variety tends to be macro crystalline and the cryptocrystalline varieties are either translucent or mostly opaque.