Quartz
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Quartz is a hard, crystalline mineral composed of silica. The atoms are linked in a continuous framework of SiO₄ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO₂. Wikipedia
Chemical formula: SiO2
Category: Silicate mineral
Cleavage: Indistinct
Color: Colorless through various colors (pink, orange, purple, dark brown) to black
Crystal class: α-quartz: trapezohedral (class 3 2); β-quartz: trapezohedral (class 6 2 2)
Crystal system: α-quartz: trigonal; β-quartz: hexagonal
Fracture: Conchoidal