Quartz is formed in many different ways and occurs in many igneous and metamorphic rocks, particularly in granite and gneiss. It is also a common gauge mineral in mineral veins and most crystals are found in this type of occurrence. With a hardness of 7, quartz is tough enough to resist weathering and is commonly found in alluvial sands and gravels.
Quartz is found throughout the world. The most abundant deposits of amethyst occur in Africa and Brazil. Madagascar, Brazil and Namibia are important sources for rock crystal. Chrysoprase occurs in Africa, Brazil and Australia. Jasper, chalcedony, and other agates are found on every continent.
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