DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Gneiss metamorphic rock. Gneiss is formed when igneous rock (such as granite) or sedimentary rock (such as sandstone or shale) is subjected to high temperatures and pressures over extended periods of time. This causes atoms in the rock to migrate, creating alternating bands of light and dark minerals. The lighter layers contain feldspar and quartz, and the darker layers are mostly composed of muscovite, biotite and hornblende. Gneiss is used for paving and building. This rounded pebble measures 160 millimetres in length.
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