Granite sample, close-up. Granite is an intrusive igneous rock consisting of the minerals quartz, feldspar and mica. This clear granite consists of the black mica biotite, a large proportion of quartz and only a little grey feldspar. The red colour is from plagioclase feldspar crystals. This sample is from the Rock of Ages Quarry, Barre, Vermont, USA, the world's largest granite quarry.