Cordierite is a cyclosilicate mineral formed by alteration in highly metamorphic rocks. It is akin to beryl, and clear crystals of appropriate color are sometimes used in jewellery. The crystals are often transected by characteristic fissures, along which replacement by other minerals takes place. That feature is called pinitization and it is clearly visible in the cordierite crystals in this polarised light thin section from a metamorphic rock.