Sapphire

Sapphire - The birthstone of September
Part of the Corundum family of minerals, blue is usually the colour associated with sapphire. They come a variety of colours though, with the second most popular colour of corundum being red, which is called Ruby. 
The other colours include pink, yellow, orange, green, purple, white and black, and in almost any intensity of these colours. 
Sapphire has a hardness of 9 on the Mohs scale, making it an excellent choice for jewellery.