Sapphire

Sapphire
The 423-carat (85 g) blue Logan Sapphire
General
Category	Oxide mineral
Formula (repeating unit)	Aluminium oxide, Al2O3
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (32/m)
Space group	R3c
Identification
Color	Typically blue, but varies
Crystal habit	As crystals, massive and granular
Fracture	Conchoidal, splintery
Mohs scale hardness	9.0
Luster	Vitreous
Specific gravity	3.95–4.03
Optical properties	Abbe number 72.2
Refractive index	nω=1.768–1.772 nε=1.760–1.763, Birefringence 0.008
Pleochroism	Strong
Melting point	2,030–2,050 °C
Fusibility	Infusible
Solubility	Insoluble
Other characteristics	Coefficient of thermal expansion (5.0–6.6)×10−6/K relative permittivity at 20 °C ε = 8.9–11.1 (anisotropic)

Coefficient of thermal expansion (5.0–6.6)×10⁻⁶/K

Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide (α-Al₂O₃). It is typically blue in color, but natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. The only color which sapphire cannot be is red - as red colored corundum is called ruby, another corundum variety. This variety in color is due to trace amounts of elements such as iron, titanium, chromium, copper, or magnesium.

Commonly, natural sapphires are cut and polished into gemstones and worn in jewelry. They also may be created synthetically in laboratories for industrial or decorative purposes in large crystal boules. Because of the remarkable hardness of sapphires – 9 on the Mohs scale (the third hardest mineral, after diamond at 10 and moissanite at 9.5) – sapphires are also used in some non-ornamental applications, such infrared optical components; high-durability windows; wristwatch crystals and movement bearings; and very thin electronic wafers, which are used as the insulating substrates of very special-purpose solid-state electronics (especially integrated circuits and GaN-based LEDs).