PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
Calcite crystals. These crystals form the scalenohedric variant of the mineral calcite (calcium carbonate). Scalenohedron crystals form the jagged pyramidal shapes seen. Calcium carbonate is a very common mineral that occurs naturally in many rocks, such as limestone and chalk. This sample, from France, is 25 centimetres long.
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