RUBY is the red variety of corundum, the second hardest natural mineral (9.0 on the Mohs scale of mineral hardness). Sapphire is the non-red variety of corundum and is generally thought to be blue, although it exists in nearly any other color. A very small amount of chromium gives corundum a pink color. Larger amounts produce stones that are a deep traffic light red, and are known as rubies.
The ruby is considered one of the four precious stones, together with the sapphire, the emerald, and the diamond