Oxide (Mineral Class)
Oxide (Mineral Class)
The mineral class behind corundum, spinel, and chrysoberyl
Oxide, in mineralogy, is the class of minerals in which metal cations are bonded directly to oxygen anions without an intermediate silicate, carbonate, or sulphate group. The class includes some of the most important gemstone species — corundum, spinel, and chrysoberyl — together with rutile, hematite, magnetite, and a number of other technically and economically significant minerals. Within the gem-quality oxides, the species are characterised by high hardness, chemical stability, and refractive indices that support strong brilliance, with trace-element substitutions producing a wide range of gem colours from ruby and sapphire to alexandrite.
Classification within the oxide class
The oxide class is subdivided in the Strunz and Dana classifications by the cation-to-anion ratio and by the structural family. The simple oxides include hematite (Fe2O3), corundum (Al2O3), and rutile (TiO2). The multiple oxides — those with two or more cation species — include the spinel group (general formula AB2O4), within which true spinel is MgAl2O4 and the chromium- and iron-bearing relatives chromite and magnetite share the same crystallographic family. Chrysoberyl (BeAl2O4) sits in its own structural family but is conventionally grouped with the gem oxides for trade purposes.
Corundum
Corundum is aluminium oxide, Al2O3. It crystallises in the trigonal system, has a hardness of 9 on the Mohs scale (the second-hardest natural material after diamond), a specific gravity of 3.95 to 4.05, and refractive indices around 1.76 to 1.78 with a birefringence of 0.008. Trace chromium produces the red colour of ruby, while combinations of iron, titanium, and other elements produce the colours of the various sapphires. Corundum is the principal heat-treatment substrate of the coloured-stone trade and is the species in which lattice diffusion of beryllium, titanium, and other elements has been most aggressively developed.
Spinel
Spinel proper is magnesium aluminium oxide, MgAl2O4, crystallising in the cubic system. It is hardness 8, specific gravity 3.6 to 3.7, and refractive index 1.71 to 1.72. Chromium produces the saturated red of fine Burmese spinel, iron and chromium together produce pinks and oranges, and cobalt produces the rare blue spinels of the Vietnamese and Sri Lankan deposits. Spinel is generally untreated, which is a significant point in its market position relative to corundum.
Chrysoberyl
Chrysoberyl is beryllium aluminium oxide, BeAl2O4, crystallising in the orthorhombic system. It is hardness 8.5, specific gravity 3.7 to 3.75, and refractive indices 1.74 to 1.75. The species hosts two of the most distinctive optical phenomena in the gem trade: the strong colour change of alexandrite, produced by chromium substitution, and the chatoyancy of cat's-eye chrysoberyl, produced by parallel inclusions. Chrysoberyl is rarely treated in the modern trade.
The minor gem oxides
Beyond the three principal species, the oxide class includes a number of minor gem materials. Rutile is occasionally cut for collectors, with a refractive index of 2.62 to 2.90 making it one of the most dispersive natural transparent materials. Anatase and brookite, both also titanium dioxide, occur as collector specimens. Hematite is widely cut as cabochon and as carving material. Magnetite and chromite occur as collector specimens but are rarely cut.
Identification and the place of oxides in the trade
The high refractive indices of the gem oxides — corundum, spinel, and chrysoberyl all sit above 1.7 — give them strong brilliance and distinguish them from the silicate species in routine gemmological identification. Their hardness makes them durable in everyday wear. The oxide class as a whole is the second most important gem mineral group after the silicates, both by total trade volume and by the value concentration of the high-end coloured-stone market: the great rubies, sapphires, and alexandrites of the world are oxides.