The traditional birthstone for September is the sapphire. Like the ruby, the birthstone for July, sapphires are mined from the mineral, corundum. In fact, sapphires, rubies, and emeralds have the same crystal and chemical formation. If the corundum is light pink to deep crimson in color, it is called a ruby. If it is green, it is called an emerald. If it is a deep, rich blue, it is called a sapphire. Corundum actually exists in all colors of the rainbow, but the red, green, and blue varieties are the most valuable.