Historically often confused with corundum (rubies and sapphires), spinel is chemically different, although its appearance and colours are similar. The central stone in England’s state crown is, in fact, spinel, although at the time it was set it was thought to have been a ruby. Distinct in both composition (hard vitreous magnesium aluminium oxide) and structure (octahedral crystals), these days spinel is acknowledged as a precious stone in its own right.