The Black Prince’s Ruby. The Timur Ruby. For centuries, spinel, the great imposter, masqueraded as ruby in Europe’s crown jewels.

Spinel, like garnet and diamond, is singly refractive, meaning it displays the same physical properties in all crystal directions. It crystallizes in the cubic system, typically forming octahedral shapes—resembling two pyramids joined at their bases. Well-formed spinel crystals are relatively common.

In addition to octahedral forms, spinel can also crystallize into flattened shapes due to a phenomenon known as twinning. In these twinned crystals, the pyramids of the octahedron rotate against each other during growth, creating a distinct appearance. Large gemstones cut from these twinned crystals often have a shallower profile and should be evaluated based on overall beauty rather than just proportions.

Spinel is a member of a group of minerals with a similar crystal structure, but not all of these minerals are transparent enough for use in jewellery. Spinel itself comes in a broad range of colours, including orange, vivid “stoplight” red, vibrant pink, and various shades of purple, blue, violet, and bluish green.

The intense red and pink hues of spinel are caused by traces of chromium, with higher chromium content leading to deeper red colours. Orange and purple spinel’s get their colours from a combination of iron and chromium. Violet to blue spinel’s are coloured by trace amounts of iron, with cobalt providing the most saturated blue tones.