Black Spinel

Spinel forms in metamorphic rocks, especially marbles and limestone contact deposits, and in their weathering products. Black spinel is still abundant at the Bo Phloi gem field in Kanchanaburi Province, western Thailand, once famous for blue sapphire. Many spinels from Sri Lanka are black, and grey to black spinels from Myanmar are occasionally cut as star stones.

Opaque black spinel has a bright vitreous luster, a refractive index and specific gravity higher than those of ordinary transparent spinel, and strong magnetism from its iron. Several cheaper black stones are sold as black spinel, including black tourmaline (lower luster and refractive index) and darkly irradiated quartz (lower refractive index, translucent at thin edges); black sapphire and black cubic zirconia can also be confused with it. Some transparent dark blue or blue-green spinel is marketed as black spinel. In Thailand the word "nin" now covers various black stones, so it does not guarantee spinel. Synthetic spinel also exists. Natural spinel is seldom treated. Full confirmation of black opaque stones may require laboratory tests such as Raman or FTIR spectroscopy.

With a hardness of 8 and no cleavage, spinel is a durable gem for rings and other jewellery. Ultrasonic and steam cleaners can be used, although fractures could pose a problem; warm soapy water is always a safe alternative. Spinel is stable when exposed to light and chemicals.

Black spinel was once referred to as ceylonite. At Bo Phloi in Thailand, after corundum production practically ceased, local gem cutters turned to the still-abundant black spinel to make their living and fashion it into various kinds of jewellery.