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Spinel

Spinel is one of a group of minerals which crystallize in the isometric system with an octahedral habit, and whose chemical compositions are analogous. These minerals are combinations of bivalent and trivalent oxides of magnesium, zinc, iron, manganese, aluminium and chromium, the general formula being represented as R''O·R'''₂O₃. The bivalent oxides may be MgO, ZnO, FeO and MnO, and the trivalent oxides may be Al₂O₃, Fe₂O₃, Mn₂O₃ and Cr₂O₃. Important members of the spinel group are:

spinel - MgAl₂O₄
gahnite (zinc spinel) - ZnAl₂O₄
franklinite - (Fe·Mn·Zn), (Fe·Mn)₂O₄
chromite - (Fe·Mg)Cr₂O₄

True spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of Myanmar and Thailand.

Spinel usually occurs in isometric crystals, octahedrons, 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 lustre. It may be colorless, but is usually various shades of red, blue, green, yellow, brown or black. There is an unique natural white spinel, now lost, that surfaced briefly in what is now Sri Lanka. Another famous spinel is the Black Prince's Ruby in the British Crown Jewels.

The name "spinel" is derived from the Greek word for spark, in reference to the fiery red color of spinels often used for gems. The transparent red spinels are also called spinel-rubies or balas-rubies and were often confused with actual rubies in ancient times. "Balas" is derived from Balascia, the ancient name for Badakhshan, a region in central Asia situated in the upper valley of the Kokcha river[?], one of the principal tributaries of the Oxus river. Yellow spinel is called rubicelle and violet-colored manganese-bearing spinel is called almandine.

Spinel is found as a metamorphic mineral, and also as a primary mineral in basic rocks, because in such magmas the absence of alkalis prevents the formation of feldspars and any aluminium oxide present will form corundum or combine with magnesia to form spinel. This is why spinel and ruby are often found together.