DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Tourmaline in gneiss. The elongated crystals (black) are deposits of the mineral tourmaline (schorl) embedded in gneiss rock. Gneiss is a metamorphic rock, one formed at high temperatures and pressures deep within the Earth's crust. The tourmaline has broken up under the pressures, forming the fragmented crystals seen here. This sample is 14 centimetres across.
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