Spinel

Spinel
Spinel: 1.83 and 4.13 ct
General
Category	Oxide minerals Spinel group Spinel structural group
Formula (repeating unit)	MgAl2O4
Strunz classification	4.BB.05
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Unit cell	a = 8.0898(9) Å; Z = 8
Identification
Color	Various; red, pink, blue, lavender/violet, dark green, brown, black, clear
Crystal habit	Octehedra or flat triangular plates caused by twinning
Twinning	common
Cleavage	None
Fracture	Conchoidal
Mohs scale hardness	7.5–8.0
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to Opaque
Specific gravity	(depending on the composition) the rare, Zn-rich spinel can be as high as 4.40, otherwise it averages from 3.58-3.61
Optical properties	Isotropic
Refractive index	1.719
Pleochroism	Absent
Solubility	none
Other characteristics	Weak to medium magnetic, sometimes fluorescent (red synthetic yes, natural red sometimes)
References

Spinel (pronunciation: /ˈspɪnɛl/) is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl₂O₄ in the cubic crystal system. Its name comes from Latin "spina" (arrow).Balas ruby is an old name for a rose-tinted variety.

Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. It has an imperfect octahedral cleavage and a conchoidal fracture. Its hardness is 8, its specific gravity is 3.5–4.1, and it is transparent to opaque with a vitreous to dull luster. It may be colorless, but is usually various shades of red, blue, green, yellow, brown, black, or (uncommon) violet. There is a unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Some spinels are among the most famous gemstones: among them are the Black Prince's Ruby and the "Timur ruby" in the British Crown Jewels, and the "Côte de Bretagne", formerly from the French Crown jewels. The Samarian Spinel is the largest known spinel in the world, weighing 500 carats (100 g).

The transparent red spinels were called spinel-rubies or balas rubies. In the past, before the arrival of modern science, spinels and rubies were equally known as rubies. After the 18th century the word ruby was only used for the red gem variety of the mineral corundum and the word spinel came to be used. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Panj River, one of the principal tributaries of the Oxus River. Mines in the Gorno Badakhshan region of Tajikistan was for centuries the main source for red and pink spinels.