The St Edward's Sapphire which is one of the oldest gemstones in the royal collection is mounted in the Maltese Cross at the top of the Imperial State Crown. It was named after Edward the Confessor King of England (1042-1066) who is believed to have worn the sapphire in a ring. Although it's believed to have been buried with him in Westminster Abbey in 1066, the ring is then believed to have been removed when his body was re-interred in 1163.
St Edward's Sapphire was cut into its present form for Charles II and was later placed into the Maltese Cross at the request of Queen Victoria.
The Imperial State crown along with the St Edward's Sapphire is currently on display in Jewel House in the Tower of London. The crown is worn by a newly appointed monarch as they leave Westminster Abbey after the coronation and during the State Opening of Parliament. It's adorned with 2,868 diamonds, 17 sapphires, 11 emeralds, 269 pearls and 4 rubies.