Mineralogists classify minerals mainly by chemistry, grouping them by the negatively charged ion or ion group that dominates their formula, and then by crystal structure. That system puts diamond with the native elements, ruby and spinel with the oxides, and emerald, garnet and topaz with the silicates. Gemologists use the same framework, adding the terms species, variety and group to name stones in a way that matches the science.
What counts as a mineral
A mineral is a naturally occurring, inorganic solid with a definite chemical composition (or a defined range of composition) and an ordered internal atomic structure. Each part of that definition rules something out:
- Naturally occurring excludes lab-grown materials. A synthetic ruby has the chemistry and structure of corundum, but it is a synthetic, not a mineral.
- Inorganic and solid excludes amber, pearls, jet and coral, which come from living things. They are called organic gems.
- A definite composition means a mineral can be written as a chemical formula, though many minerals allow some substitution of one element for another.
- An ordered atomic structure means the atoms form a crystal lattice. Natural solids without one, such as opal and obsidian, are usually called mineraloids.
The International Mineralogical Association (IMA) approves new mineral species and names. Several thousand species have been approved, and only a small share of them are ever cut as gems.
Rocks are a separate category. A rock is an aggregate of one or more minerals. Lapis lazuli is a rock made mostly of lazurite with calcite and pyrite, and many jaspers and some jade are rocks too.
The chemical classes
The classification most mineralogists use follows the Dana and Strunz systems, which sort minerals by their main anion or anion group. The classes below cover nearly every gem.
| Class | Defining chemistry | Gem examples |
|---|---|---|
| Native elements | A single element, not combined | Diamond (carbon), gold, silver, platinum, copper |
| Sulfides | Metals combined with sulfur | Pyrite, sphalerite, marcasite |
| Oxides and hydroxides | Metals combined with oxygen | Corundum (ruby, sapphire), spinel, chrysoberyl, hematite, rutile |
| Halides | Metals combined with fluorine, chlorine and similar | Fluorite |
| Carbonates | Carbonate group (CO3) | Calcite, aragonite, malachite, azurite, rhodochrosite |
| Sulfates | Sulfate group (SO4) | Barite, gypsum (selenite, alabaster) |
| Phosphates | Phosphate group (PO4) | Apatite, turquoise, variscite, amblygonite |
| Silicates | Silicon and oxygen (SiO4 building blocks) | Quartz, beryl, garnet, tourmaline, topaz, feldspar, jade |
Some systems place quartz with the oxides because its formula is SiO2. Most modern references place it among the silicates because of its framework structure.
Native elements
Diamond is pure carbon, the only major gemstone made of a single element. The precious metals gold, silver and platinum are also native elements, which is why they turn up as nuggets in river gravels.
Oxides
Oxides include some of the hardest gems. Corundum, aluminum oxide, is the mineral behind ruby and sapphire, with a hardness of 9. Spinel, a magnesium aluminum oxide, and chrysoberyl, a beryllium aluminum oxide, are oxides as well.
Carbonates and phosphates
Carbonates tend to be soft and react with acid. Malachite is a copper carbonate, and the aragonite in pearl nacre is calcium carbonate. Phosphates include apatite, the reference mineral for 5 on the Mohs scale, and turquoise, a copper aluminum phosphate.
Silicates and their subclasses
Silicates are the largest class and make up most of Earth's crust. Their building block is the silicate tetrahedron, one silicon atom surrounded by four oxygen atoms. How those tetrahedra link together defines six subclasses, and the linkage affects hardness, cleavage and crystal shape.
| Subclass | How tetrahedra link | Gem examples |
|---|---|---|
| Nesosilicates (island) | Isolated tetrahedra | Olivine (peridot), garnet, zircon, topaz, kyanite, andalusite |
| Sorosilicates (paired) | Pairs of tetrahedra | Epidote, zoisite (tanzanite) |
| Cyclosilicates (ring) | Rings of tetrahedra | Beryl, tourmaline, cordierite (iolite) |
| Inosilicates (chain) | Single or double chains | Pyroxenes (jadeite, diopside, spodumene); amphiboles (nephrite) |
| Phyllosilicates (sheet) | Flat sheets | Micas, serpentine, talc, chrysocolla |
| Tektosilicates (framework) | Three-dimensional frameworks | Quartz, feldspars (moonstone, labradorite, sunstone), lazurite |
Ring silicates explain the long six-sided prisms of beryl, where stacked rings form channels along the crystal. Sheet silicates such as mica split into thin flakes along their sheets. Jade's toughness comes from tightly interwoven crystals of chain silicates, jadeite (a pyroxene) or nephrite (an amphibole).
Species, variety and group
Gemology adds its own naming layers on top of mineral classes.
A species is a single mineral with its own composition and structure, such as corundum, beryl or quartz.
A variety is a type within a species, usually defined by color or optical effect. Ruby is the red variety of corundum, and sapphire covers the other colors. Emerald, aquamarine and morganite are varieties of beryl. Amethyst and citrine are varieties of quartz. Varieties share the same physical and optical properties, so they cannot be separated by refractive index or specific gravity alone. The difference usually comes from trace elements, as explained on the gemstone color page.
A group is a family of related species that share a crystal structure but differ in chemistry. Garnet is the clearest example. Pyrope, almandine, spessartine, grossular, andradite and uvarovite are separate species that blend into one another through substitution, so many gem garnets are mixtures. Tourmaline is another group: elbaite, schorl, dravite and liddicoatite are species within it. The feldspars form a group as well.
A series is a continuous run of compositions between two end members, such as the pyrope to almandine range in garnet or the albite to anorthite range in plagioclase feldspar.
Why classification matters for gems
Knowing a gem's species and class tells you a lot before any test. Oxides like corundum and spinel tend to be hard and durable. Carbonates are soft and sensitive to acids, so jewelry cleaning methods must be gentle. Species with strong cleavage, like topaz and spodumene, need care in setting.
Classification also guides identification. Two red stones might be ruby (an oxide) and garnet (a silicate). They belong to different species, with different refractive indices, specific gravities and optical character, which is how a lab tells them apart.
Frequently asked questions
Is a gemstone always a mineral?
No. Most gemstones are minerals, but opal is a mineraloid, lapis lazuli is a rock, and amber, pearl and coral are organic materials.
What is the difference between a species and a variety?
A species is a distinct mineral, such as beryl. A variety is a color or effect type within that species, such as emerald or aquamarine.
Why is garnet called a group?
Garnet includes several mineral species with the same crystal structure but different chemistry. They mix freely, so a single garnet can be part pyrope and part almandine.
Is diamond a mineral?
Yes. Natural diamond is a native element mineral made of carbon. A lab-grown diamond has the same composition and structure but is a synthetic, not a mineral.