Sapphire
Sapphire is the gem-grade variety of corundum (aluminum oxide, Al2O3) in any color other than red. Blue is the iconic hue, but pink, yellow, green, and color-change variants are all valued.
About Sapphireextended article
Blue (and other-color) variety of corundum. Blue from Fe-Ti intervalent charge transfer.
Full mineralogical data
Sapphire is a named variety, not a separate species. For complete data on chemistry, crystal system, hardness, optics, paragenesis, treatments, and inclusions — refer to the parent species:
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About Sapphire
Sapphire is corundum, the second-hardest natural mineral after diamond. Its blue color comes from trace titanium and iron substituting for aluminum in the crystal lattice. Sapphire forms in metamorphic and igneous rocks and weathers into alluvial gem gravels in tropical settings. Major sources include Kashmir (historic and exhausted), Burma (Mogok), Sri Lanka, Madagascar, Australia, Thailand, and Montana.
Identification & care
Sapphire ranks 9 on the Mohs scale with a specific gravity around 3.99. It crystallizes in the trigonal system as barrel-shaped or tabular crystals with hexagonal cross-section. Luster is vitreous to adamantine, no cleavage, conchoidal fracture. Doubly refractive (uniaxial negative) with a refractive index of 1.76 to 1.77. The combination of extreme hardness, density, and characteristic crystal form distinguishes it from blue spinel and iolite.
Collector context
Collector notes
Mineral specimens of sapphire — as opposed to faceted gems — are highly collectible when they retain crystal form on matrix. Kashmir cornflower-blue material is essentially unobtainable; Sri Lankan and Madagascan crystals are the most accessible high-quality specimens. Heating to improve color is universal in the gem trade but reduces specimen value when disclosed.