Reference work entry
A spinel gem is spinel with gem-grade quality. It mainly refers to magnesia-alumina spinel, which is a magnesium-aluminium oxide (MgAl 2O 4). It has a Mohs hardness of 8 and a high refractive index. It is easily confused with ruby and red garnet. It has many colours and can be colourless, pink, red, purple, light purple, bluish purple, blue, yellow and brown. Some spinel can have asterism (star effect), and a rare and precious variety appears blue in daylight but violet under an incandescent lamp (Fig. 38).
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