quartz

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quartz Rock-forming mineral, the natural form of silicon dioxide (silica), SiO2. It is widely distributed, occurring in igneous and metamorphic rocks (notably granite and gneiss), and in clastic sediments. It is also found in mineral veins. It forms six-sided crystals. Pure quartz is clear and colourless but the mineral may be coloured by impurities. The most common varieties are colourless quartz (rock crystal), rose, yellow, milky, and smoky. The most usual cryptocrystalline varieties, whose crystals can be seen only under a microscope, are chalcedony and flint. Quartz crystals exhibit the piezoelectric effect and are used in electronic clocks and watches to keep accurate time. Hardness 7; r.d. 2.65.

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