Spinel comes in red, pink, purple and blue. Over the past few centuries, many emperors and monarchs confused red spinel for ruby. For example, the famous 14th century Black Prince’s Ruby placed on the British Imperial Crown is actually spinel, not ruby as the name suggests. Nevertheless, because of its beauty and stark similarities to ruby and sapphire, the gem has traditionally been treasured. Like diamonds, the gem is singly refractive.