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Melaphyre igneous rock. This rock consists of isolated feldspar mineral crystals held within dark, fine-grained igneous rock. Igneous rock is rock that has solidified from molten magma. This pattern of large mineral crystals within a matrix is known as porphyritic texture. This sample measures 110 millimetres across. (KEYSTONE/SCIENCE PHOTO LIBRARY/CHR 366 DIRK WIERSMA/SCIENCE PHOTO LIBRARY) === ===