Sapphires are the blue variety of the precious gemstone corundum. They can be found in a wide range of shades, from pale blue to almost black; however, the most prized shade of sapphire is cornflower blue, which is also known as Kashmir sapphire.
The chemical composition for sapphires is Al2O3, or aluminum oxide. It measures a 9 on the Moh’s scale, which is a system for establishing the hardness of gemstones and minerals. The only gem harder than a sapphire is a diamond.
The star in a star sapphire is the result of microscopic inclusions of another mineral, usually rutile. These inclusions cause an effect known as asterism—when the stone is cut along its C axis, a star appears. The stars can have as few as two or as many as 12 rays, which may also be of varying intensity.
Sapphires are the birthstone for the month of September. In addition, star sapphires are thought to ward off mental illness and colic, among other ailments. Due to the presence of the star, they are considered to be stones that connect the wearer to a higher plane of consciousness.