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-colour twinned crystals are typically shallow, and should be judged on their overall beauty rather than on proportions alone.
The spinel used in jewellery is a small part of a group of minerals that share the same crystal structure. Not all of them form transparent crystals suitable for jewellery use, however. Spinel offers a range of hues, from orange to intense “traffic light” red, vibrant pink and all shades of purple, blue and violet through to 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 colour to a mixture of iron and chromium.
Violet to blue spinel can be coloured by trace amounts of iron, and vibrant blues owe their saturated colour to trace amounts of cobalt.