Sapphire

Sapphire
The 423-carat (85 g) blue Logan sapphire
General
Category	Oxide mineral
Formula (repeating unit)	aluminium oxide, Al2O3
Identification
Color	Often Blue but varies
Crystal habit	massive an granular
Crystal seestem	Trigonal Seembol (32/m) Space Group: R3c
Cleavage	none
Fractur	conchoidal, splintery
Mohs scale hairdness	9.0
Luster	vitreous
Streak	white
Specific gravity	3.95–4.03
Optical properties	Abbe nummer 72.2
Refractive index	nω=1.768–1.772 nε=1.760–1.763, Birefringence 0.008
Pleochroism	Strang
Meltin pynt	2030–2050 °C
Fusibility	infusible
Solubility	Insoluble
Ither characteristics	coefficient o thermal expansion (5.0–6.6)×10−6/K

Sapphire (Greek: σάπφειρος; sappheiros, 'blue stane', which probably referred instead at the time tae lapis lazuli) is a gemstane variety o the mineral corundum, an aluminium oxide (α-Al₂O₃). Trace amoonts o ither elements such as airn, titanium, chromium, copper, or magnesium can gie corundum blue, yellae, purpie, orange, or a greenish colour. Chromium impurities in corundum yield a pink or reid tint, the latter bein cried a ruby.