Spinel is the magnesium aluminum member of the larger spinel group of minerals. 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, blue, green, yellow, brown, or black.
Black coating uses either ruthenium or TIN cobalt black to give a piece of jewelry a light black or completely black appearance. It is very durable and resistant to scratches. Black plating gives the jewelry piece a sense of masculinity and high-tech.