Sapphire is a crystal form of the mineral corundum containing minute traces of iron and titanium to cause the colour blue. In a similar way, the more iron the corundum contains, the darker the blue. While the finest sapphires in the world are usually from Kashmir, India, there are other localities which produce high-quality sapphires too. Burmese stones are normally of a deeper, richer, more intense hue sometimes called ‘royal blue’. Their Ceylon or Sri Lankan counterparts, by contrast, tend towards paler, brighter colours. The most famous sapphire in the world is the 12 carats oval Ceylon sapphire featured as the centre stone of the engagement ring of Katherine Middleton and Prince William – previously worn by Princess Diana.