In geology and mineralogy, a mineral variety is a subset of a mineral species or mineraloid distinguished by a special feature such as a particular impurity, structural defect, colour, habit or locality. A variety is not a separate mineral species but an identifiable type within a species, produced by small compositional or structural differences.
Amethyst, for example, is a variety of quartz whose purple colour partly reflects iron impurities and colour centres produced by irradiation. Smoky quartz is a brown variety of quartz associated with aluminium impurities and natural radiation. Precious opal is a variety of opal whose iridescence comes from the ordered stacking of silica spheres. Blue John is a banded variety of fluorite characterised by purple, blue and yellow bands.
Mineral varieties may be further divided into sub-varieties. Sub-varieties are commonly distinguished by colour intensity, banding style, inclusion type or locality, but these distinctions are used mainly in gemmology, collecting and commerce and do not have formal status in International Mineralogical Association nomenclature.
Unlike mineral species, mineral varieties are not defined or named by the IMA. The IMA approves and names new mineral species, whereas variety names usually come from historical tradition, mining practice, the gem trade or local usage. A variety may therefore have several names, and one name may refer to different varieties of different species.
A mineral variety differs from a mineral group. A group is a set of structurally related species with a continuous range of composition, such as the olivine, amphibole and tourmaline groups, whereas a variety is a subtype within a single species. A variety also differs from a mineraloid: mineraloids such as opal and obsidian lack a definite crystal structure, whereas a variety still belongs to a species with a definite structure.
Varieties also differ from solid-solution end members. End members may be named as separate species, such as forsterite and fayalite, whereas varieties normally do not cross species boundaries and vary within the composition range of a species. Corundum containing trace chromium is red and called ruby, and corundum containing iron and titanium is blue and called sapphire, but both are varieties of corundum rather than separate species.
Variety names are important in gemmology, collecting and trade. Ruby, sapphire, emerald, aquamarine, amethyst, citrine, smoky quartz, precious opal and Blue John are variety or gem-trade names rather than IMA species names. These names affect price, cutting choices and identification methods, so gem laboratories normally report both the species name and the variety or trade name.
In scientific writing, variety names should be used cautiously. Researchers normally give the species name first and then note the variety or compositional feature, so that a commercial name is not mistaken for a taxonomic unit. For varieties with important compositional or structural significance, such as chromium-bearing corundum, manganese-bearing calcite or rare-earth-bearing fluorite, variety information can provide clues to genesis, provenance or mineralising conditions.
Mineral varieties record trace elements, irradiation, redox conditions and growth environments at the time of formation. Analysis of colour origin, inclusion suites and isotopic composition can be used to infer deposit type, fluid source and later alteration history. Varieties are also important references for mineral materials design: adding specific elements to corundum, spinel and fluorite can produce laser, fluorescence or gemmological effects.
In museums and collections, varieties are a principal way of displaying mineral diversity. Different colours, habits and localities of the same species can be exhibited together to help the public understand the hierarchy between mineral species and varieties. Varieties are therefore both a marginal taxonomic concept and an important bridge between science, craft and commerce.