Quartz. A cluster of clear quartz crystals (rock crystals) against a black background. Quartz (silicon oxide) 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 of the earth's crust (12 percent by volume).
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