PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Fluorite crystals. Fluorite, or fluorspar (calcium fluoride) is a fairly common mineral, extensively mined for use in steelmaking (as a flux) and in other industrial processes. It occurs in cubic crystal groups that can vary in colour from soft green and yellow to colourless and deep purple. This sample, from France, is 12 centimetres wide.
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