Sapphire and ruby are the main two gemstone varieties of the mineral corundum. Almost identical from a geological perspective, the main difference between the two is their colour. Whilst red corundum is known as ruby, all other colours are known as sapphire. Although found in several different colours pink and blue sapphires tend to be the most renowned.
Sapphires are far more abundant than rubies due to the larger occurrence of chromium, iron, and titanium which is the cause of their colour. Corundum which is the second hardest mineral on Earth is a naturally occurring crystalline form of aluminium oxide which is a chemical compound of aluminium and oxygen.
Birthstone for the month of September, the name sapphire comes from the Greek word 'sappheiros' meaning "blue stone" but it's generally believed that over time the name was misused and that it originally referred to the blue coloured metamorphic rock that's best known as lapis lazuli.
Sapphires have been associated with royalty and nobility throughout history and both Princess Anne and Diana Princess of Wales were given engagement rings that featured a blue sapphire. Another two famous sapphires are the Stuart Sapphire and St Edward's Sapphire both of which are part of the British Crown Jewels. The Logan Sapphire which is one of the largest faceted gem-grade blue sapphires in existence is on display in the National Museum of Natural History in Washington D.C. Other famous stones include the Black Star of Queensland which before the discovery of the Star of Adam was the largest star sapphire ever to have been mined. The Star of Adam was found in Sri Lanka in 2015 and its name comes from a Muslim belief that Adam went to Sri Lanka after being expelled from the Garden of Eden and lived out his days on Adam's Peak which is a well known mountain not far from the city of Ratnapura. The Star of India which was also mined in Sri Lanka is currently housed in the American Museum of Natural History in New York.
Star sapphires exhibit a star-like optical phenomenon known as asterism which is caused as light reflects of needle-like rutile inclusions within the stone. Rutile is a natural mineral that's made up primarily of titanium dioxide. In order for asterism to be seen the gemstone must be carefully cut and polished as a cabochon.
Many of the world's finest sapphires are heat treated in order to enhance colour and improve clarity but this practice is not new and can be traced back at least as far as Ancient Rome. Sapphires and in particular rubies which have not been heat treated are considered to be extremely unusual.
Although Sri Lanka and Madagascar are currently the world's largest exporters, sapphires can also be found in Myanmar (formerly Burma), Australia, Thailand, India, Pakistan, Afghanistan, Tanzania, Kenya and China.
In crystal healing the sapphire is believed to improve concentration and mental clarity and help focus the mind. It has the ability to find solutions to problems and is said to remove mental tension bringing inner peace and serenity.
Graded 9 on Mohs scale of mineral hardness sapphire is believed to have been one of the stones in the Breastplate of the High Priest, a biblical garment worn by the Jewish high priest when presenting himself to God.
Wikipedia on Sapphire