Spinel

This large green crystal is a spinel. Spinel crystals consist of magnesium aluminium oxide and naturally occur in variety of colours, due to trace amounts of other elements in the structure. This particular crystal is a chlorospinel, with its characteristic green appearance arising due to trace amounts of copper and iron. Bright red spinels are the most valuable due to their similarity in appearance to the ruby. In fact, many old rubies have been found to be spinels, including the Black Prince’s Ruby that forms the centrepiece of the Royal Crown of Great Britain. Curiously, despite red spinels being much rarer than real rubies, they are less valuable than rubies.