Quartz occurs in many varieties. Rock crystal is colorless Quartz and sub-varieties include ghost quartz, in which growth stages are marked by inclusion, and rutilated quartz (sagenite) which contain hair-like rods of rutile. Amethyst is a type of quartz that gets its color from iron. Amethyst crystals are formed in large geodes that are found in rock cavities created by volcanic gas bubbles and are bound between granite intrusions and volcanic sediments