High pressures and temperatures within the Earth's crust can change the textures and chemistry of minerals. In some cases, the high temperatures and pressures force traces of water or carbon dioxide out of a mineral, transforming it into a less hydrated species. Sometimes a mineral will not change its chemical composition during metamorphism, but its crystal structure will change into a more compact arrangement. For example, extreme heat and pressure can convert plain carbon from dull graphite, which has a flat, hexagonal crystal structure, to brilliant diamond, which has a closely packed, cubic crystal structure. Scientists say a metamorphic reaction is isochemical if minerals have the same amounts of chemical elements before and after the reaction (iso comes from the Greek word for equal).

Minerals can be opaque, meaning they block all light from passing through them; translucent, meaning they block some of the light; or transparent, meaning they pass most or all the light. A typical garnet or amethyst crystal is translucent; if you hold it up to a bright light, only a small fraction of the light entering the crystal ever reaches your eye, and you cannot see clear images through the crystal. Mica, a silicate mineral, can be cut into thin sheets so transparent that they serve as panes of a window. One colorless, transparent variety of calcite, dubbed "Iceland spar," exhibits a phenomenon called double refraction, which makes one object look like two. Another mineral, a borate called ulexite, occurs in thin parallel fibers that conduct light through them by total internal reflection, just like manufactured optical fibers. Ulexite seems to project an image onto the polished surface of the mineral, giving it the nickname "television stone."