Quartz is a common rock-forming mineral with the composition SiO2. It crystallizes in a hexagonal crystal system, where the crystals are in the form of hexagonal columns. The column surfaces have transverse growth striations, and crystal twinning is common. It always appears in druse, in granular clusters or in blocky aggregates. The colours range from white, grey, red, purple, pink, yellow, green, and brown to black. It is often colourless and transparent with white streaks. It has transparent to translucent diaphaneity, and the fresh cleavage surfaces are vitreous, but the fractures have a greasy lustre. Its specific gravity is 2.65, its Mohs hardness is 7, and the fractures are conchoidal to uneven. Quartz usually forms in magmatic rocks, metamorphic rocks and sedimentary rocks, and it is sometimes found in ore veins containing metallic minerals.