Spinel is a magnesium aluminum oxide, occurring in many colors with excellent durability. Natural spinel is really a group of related minerals in which its magnesium is substituted with other elements, like iron, manganese and zinc. Spinel is found in areas that are famous for sapphire deposits. For many centuries, most gem spinels were misidentified as sapphires or rubies because they have similar properties and occur in the same geological environments.
Spinel, like garnet and diamond, is singly refractive, with the same physical properties in all crystal directions. It belongs to the cubic crystal system, and its characteristic crystal shape is an octahedron, which looks like two back-to-back pyramids. Well-formed spinel crystals are fairly common in nature.
Spinel offers a range of hues, from orange to intense “stoplight” red, vibrant pink, and all shades of purple, blue, and violet through bluish green. Intense reds and pinks are caused by traces of chromium. The higher the chromium content, the stronger the red hue. Orange and purple stones owe their color to a mixture of iron and chromium. Violet to blue spinel can be colored by trace amounts of iron, and vibrant blues owe their saturated color to trace amounts of cobalt.
Spinel was recently added as an August birthstone, sharing this month with peridot and sardonyx. It has long been mistaken for ruby by emperors and monarchs. Many of the famous “rubies” of history were actually spinels.