This singly refractive gemstone boasts a variety of colours from dark blue to red and pinkish tones dependent on the presence of chromium and iron. Their crystal shape is also spiky and angular, thought to be the reason for its name deriving from the Latin 'spina' for thorn. Spinels are often mistaken for rubies as their composition is hugely similar but there difference is mainly seen in the hardness of the stones, as measured on the Mohs scale. The most infamous slip up of this mismatch has to be the spinel mistakenly set into the British Imperial Crown jewels - the so-called Black Prince's Ruby. Given spinel's hardness, it is extremely durable when set in jewellery.