Quartz has a fixed chemical composition, SiO2, arranged in a hexagonal pattern and referred to as a pure substance. A magnesium silicate, Mg2SiO4, is at one extreme, while the iron silicate, Fe2SiO4, is at the other extreme. The iron and magnesium are close enough in size to substitute for one another in the crystal lattice. The more iron rich variety is found in gem quality and termed peridot. A characteristic crystalline structure means there is an internal structural framework in which atoms are arranged in a geometric pattern.
Inorganic origin excludes formation by organisms.
Naturally occurring material that is lacking a portion of the mineral definition can be called mineraloid. Opal is a mineraloid because the crystalline structure is not well defined, whereas amber is a mineraloid because of its organic origin. Although most pearls are created by bivalve mollusks, some are formed in caves as calcium carbonate precipitates around bat bones or some other small grains on the floor of the cave. These classifications can be based on color, color distribution, transparency, optical phenomena, or any combination. Examples of subdivisions based on color distribution are onyx and agate.