SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY SINCLAIR STAMMERS / SCIENCE PHOTO LIBRARY
Quartz crystals, the most abundant and common mineral consisting of crystalline silica (silicon dioxide). The well-formed crystals of quartz are usually pyramidal in shape; this structure has a hardness of 7 on the Mohs' scale. Pure forms are colourless and known as "rock crystals" (as here); those varieties which contain impurities tend to be coloured. Quartz occurs in many rocks especially igneous (e.g granite and quartzite), metamorphic and sedimentary rocks. The mineral is strongly piezoelectric and is therefore used to make oscillators for clocks, radios and radar equipment; it is also used in glass, glaze and abrasives.
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