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 (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. Read more about quartz here.
The uncrystallized version of quartz is called chalcedony ("kal-SED-a-nee"). A hydrated form of silica is called opal, most of which does not resemble the gemstone.
Other Primary Minerals
Other Hydrothermal Vein Minerals
Other Metamorphic Minerals
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