Porphyritic granite rock sample. Granite is an igneous rock, one that formed from the cooling of molten rock. In this case, large crystals have formed so the rock is termed porphyritic. Granite is mostly silica (silicon dioxide) and aluminium oxide, with traces of other chemicals. The exact colour and appearance depends on these traces and the conditions under which the rock formed. One use of cut and polished porphyritic granite is as ornamental stonework for buildings, such as for kitchen surface. This sample is 9 by 8 centimetres in size.
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