Description :
The spinels are any of a class of minerals of general
formulation A2+B23+O42- which crystallise in the cubic (isometric) crystal
system, with the oxide anions arranged in a cubic close-packed lattice and the
cations A and B occupying some or all of the octahedral and tetrahedral sites
in the lattice. A and B can be divalent, trivalent, or quadrivalent cations,
including magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and
silicon. Although the anion is normally oxide, structures are also known for
the rest of the chalcogenides.
Physical Properties :
Its hardness is 8, its specific gravity is 3.4-4.1 and
it is transparent to opaque with a vitreous to dull lustre. It may be
colorless, but is usually various shades of red, green, blue, yellow, brown or
black. Some spinels are among the most famous gemstones: Among them is the
Black Prince's Ruby and the 'Timur ruby' in the British Crown Jewels, and the
'cote de Bretagne' formerly from the French Crown jewels. The Samarian Spinel
is the largest known spinel in the world, weighing 500carats (100g). 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 became used.The ancient
name for Badakhshan, a region in central Asia resided in the upper valley of
the Kokcha River, one of the principal tributaries of the Oxus River. The
Badakshan province was for centuries the main source for red and pink spinels.
Natural Occurrence :
Sri Lanka, Thailand, Burma and Afghanistan.