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Quartz
Silicon dioxide SiO2. Quartz is the most abundant and 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 macrocrystalline, which is individual crystal visible to the unaided eye, and the other is microcrystalline or cryptodrystalline varieties, aggregates of crysstals visible only under high magnification. Chalcedony is the generic term for cryptocrystalline quartz. The transparent variety tends to be macrocrystalline and the cryptocrystalline varieties are either translucent or mostly opaque.