Quartz crystals. These are also known as rock crystals. Quartz (silicon dioxide) is colourless when pure, but the presence of impurities can give a ra

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Quartz crystals. These are also known as rock crystals. Quartz (silicon dioxide) is colourless when pure, but the presence of impurities can give a range of colours including white, pink, yellow, blue, green and smoky brown. The colourless variety is highly prized by collectors. Quartz is one of the commonest minerals in the Earth's crust (12% by volume).

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Keywords: crystal, crystalline, crystals, dioxide, gem, geology, mineral, quartz, rock, semi-precious, silicon, stone