spinel 34 mb portrait

Spinel crystallizes in the isometric system; common crystal forms are octahedra, usually twinned. It has no true cleavage, but shows an octahedral parting and a conchoidal fracture.[8] 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, lavender, blue, green, brown, black, or yellow.[9] Chromium(III) causes the red color in spinel from Burma.[10] 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,[11] and the "Côte de Bretagne", formerly from the French Crown jewels.[12] The Samarian Spinel is the largest known spinel in the world, weighing 500 carats (100 g).[13]