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
Twinning	Both growth twins (in various orientations) and polysynthetic glide twinning on the rhombohedron [1011
Cleavage	Poor
Fracture	Conchoidal, splintery
Mohs scale hardness	9.0
Luster	Vitreous
Streak	Colorless
Diaphaneity	Transparent to nearly opaque
Specific gravity	3.98–4.06
Optical properties	Uniaxial (–), 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 [citation needed] relative permittivity at 20 °C ε = 8.9–11.1 (anisotropic)[1]

Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide (α-Al₂O₃) with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name sapphire is derived from the Latin word sapphirus, itself from the Greek word sappheiros (σάπφειρος), which referred to lapis lazuli.^[2] It is typically blue, but natural "fancy" sapphires also occur in yellow, purple, orange, and green colors; "parti sapphires" show two or more colors. Red corundum stones also occur, but are called rubies rather than sapphires.^[3] Pink-colored corundum may be classified either as ruby or sapphire depending on the locale. 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 as infrared optical components, high-durability windows, wristwatch crystals and movement bearings, and very thin electronic wafers, which are used as the insulating substrates of special-purpose solid-state electronics such as integrated circuits and GaN-based blue LEDs. Sapphire is the birthstone for September and the gem of the 45th anniversary. A sapphire jubilee occurs after 65 years.^[4]