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QuartzQuartz is the most abundant mineral on Earth (about 12% vol.). It has an hexagonal crystal structure made of trigonal-crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on the Mohs scale. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally a bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow pebble (usually an approximatly spherical shape), its core lined with a bed of crystals. Being one of the world's most common minerals, quartz goes by a bewildering array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Although many of the varietal names historically arose from the colour of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Colour is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true, however. Major varieties of quartz include: Chalcedony, Agate, Onyx, Jasper, Aventurine, Tiger's eye, Rock crystal, Amethyst, Citrine, Rose quartz, Milk quartz, Smoky quartz, Morion and Carnelian. |