Quartz (crystalline silica or SiO2) is the most common single mineral of the continental crust. It is unusually hard for a white/clear mineral, hardness 7 on the Mohs scale. Quartz has a glassy appearance, known formally as a vitreous luster. It never breaks in splinters but fractures in chips with a typical shell-shaped or conchoidal surface. Once familiar with its appearance and range of colors, even beginner rockhounds can reliably identify quartz by eye or, if necessary, with a simple scratch test. It is so common in coarse-grained igneous rocks and metamorphic rocks that its absence may be more noteworthy than its presence. And quartz is the main mineral of sand and sandstone.
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