Polished marble Like many igneous and metamorphic rocks, marble can be ground and polished smooth. Granite is a coarse-grained and fairly acid igneous rock found in batholiths and other large intrusions. It contains pink or pearly feldspar, black mica and glassy grey quartz. Dolerite is a medium-grained, basic igneous rock found in small intrusions. Basalt is a fine-grained, basic igneous rock often forming in thick sheets covering vast areas, like the Deccan of India.
Contact metamorphism is when rock is scorched by the heat of an igneous intrusion.
This turns sandstone into quartzite, limestone into marble and mudstone and shale into hornfels. Regional metamorphism is when rocks are crushed beneath mountains by the moving plates of the Earth's surface. This turns mudstone and shale to slate, then to schist and then to gneiss as it is squeezed harder and harder. Schist is formed when slate made from shale is crushed even harder. It contains fine black grains of mica and, occasionally, gems like garnet.