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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