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Quartz
Quartz is one of the most common minerals in the Earth's continental crust. It belongs to the hexagonal crystal system, and is made up of silica (SiO2) tetrahedra. Quartz has a hardness of 7 on the Mohs scale. Density is 2.65 g/cm³. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often twinned, 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 rock (usually with an approximately spherical shape) with a core lined with a bed of crystals.
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