KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY KAJ R. SVENSSON / SCIENCE PHOTO LIBRARY
Granite. Cut and polished surface of blue pearl granite (Alaskan bluestone) showing its coarsely crystalline structure. The different coloured fragments are crystals of the various minerals that make granite - mostly feldspar, mica and quartz. The blue colour comes from chemical impur- ities in some of the minerals. An igneous rock, granite is formed when molten "magma" (like lava) rises from deep in the Earth and becomes trapped underground. The magma cools down and solidifies slowly, allowing large crystals to grow. Granite's strength makes it an ideal building stone, both polished in slabs for facings and crushed to make concrete. This sample is from Alaska, USA.
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