Sapphire, the September birthstone, has been popular since the Middle Ages and, according to folklore, will protect your loved ones from envy and harm. Medieval clergy wore sapphires to symbolize heaven, while commoners thought the gem attracted heavenly blessings. Blue sapphires range from very light to very dark greenish or violetish blue, as well as various shades of pure blue. The most prized colours are a medium to medium dark blue or slightly violetish blue. Sapphire is a variety of the gem species corundum and occurs in all colours of the rainbow. Pink, purple, green, orange, or yellow corundum are known by their colour (pink sapphire, green sapphire). Ruby is the red variety of corundum.
Information taken from http://www.americangemsociety.org