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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 can also form flattened crystals that look radically different from octahedral crystals. The flattened shape occurs when the pyramids that form an octahedron rotate against each other during growth. Scientists describe this as a “twinned crystal.” Large gems cut from good-color twinned crystals are typically shallow, and should be judged on their overall beauty rather than on proportions alone.

The spinel used in jewelry is a small part of a group of minerals that share the same crystal structure. Not all of them form transparent crystals suitable for jewelry use, however. 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.