Magnesium aluminium oxide. The spinels are an extensive group of minerals, in which Mg may be replaced by Fe, Zn and above all, Mn; and Al may be replaced by Fe or Cr. The hardness and fine colors of gem quality spinel (also known as noble spinel) distinguish it as a gem material from other types of spinel.
Crystal system Cubic.
Appearance It normally occurs as distinct octahedra, as clusters also of octahedral habit, or as characteristic twins. The crystals are often isolated, sometimes aggregated, fairly complete, lustrous, and with a color varying from red to pink, violet red, pale lilac, violet blue, blue, or black. The possible color range is almost as extensive as that of corundum. It can be found as rolled pebbles in alluvium.
Physical properties Spinel has a hardness of 8. It is singly refractive, with an index of 1.71 or a little more. The density is 3.58-3.61 g/cm“.
Genesis Spinel is formed by regional metamorphism or contact metamorphism in limestones, and also schists. It is also found in pegmatitic, pneumatolytic environments, together with corundum, and in alluvial deposits.
Occurrence lt mainly occurs in Afghanistan, India, Burma, or the gem gravels of Sri Lanka, Madagascar, and Brazil;