Sapphire is the birthstone for September. Although sapphire usually refers to the blue variety of corundum (ruby is the red variety), sapphire comes in a rainbow of other colors. All red, gem-quality corundum gems are considered rubies. All other colors of gem-quality corundum are considered sapphires. 

The September birthstone is relatively hard, ranking 9 on the Mohs scale. It is a durable gemstone which makes a great choice for rings and other mountings subject to daily wear. It is an alternative to diamond when choosing a colored stone engagement ring. 
The most highly valued blue sapphires are velvety blue to violet blue, in medium to medium dark tones. Preferred sapphires also have strong to vivid color saturation. The saturation should be as strong as possible without darkening the color and compromising brightness.Sapphires is commonly heat treated to improve color or clarity which is a permanent treatment.

Sapphires can display asterism or the “star effect” due to rutile inclusions in their hexagonal crystal matrix. If this rutile is sufficiently abundant and precisely arranged, proper cabochon cutting can create six-rayed star sapphires.