Garnets in micaschist, polarized light micrograph. This is a thin section taken in transmitted light, a technique that reveals the diffraction colours of the micaschist, but leaves the garnets as black areas as they are opaque. The garnets are undeformed porphyroblasts 'floating' in a highly deformed micaschist. This type of rock results from high-grade metamorphism, the reforming of rocks after being acted on by heat and pressure at great depths. This sample is 3.2 centimetres across.