Chemically, it is silicon dioxide, SiO 2 . Varieties are classified as crystalline and cryptocrystalline. Varieties of cryptocrystalline quartz, the crystal structure of which can be seen only under the microscope, if at all, are chalcedony , flint , hornstone, and chert. Colored varieties of chalcedony known by special names are carnelian , sard, chrysoprase, agate , onyx , sardonyx, and jasper .
Clastic quartzes are sand and sandstone .
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