MARTIN LAND / SCIENCE PHOTO LIBRARY MARTIN LAND / SCIENCE PHOTO LIBRARY
A sample of amethyst showing the typical six-sided crystal prisms. Amethyst is a type of quartz crystal which is purple in colour due to the presence of trace amounts of ferric iron. 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 known as rock crystal and 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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