1. Native aluminum oxide, the hardest naturally occurring mineral known apart from diamond (corundum rates 9 on the Mohs scale of hardness).

A lack of cleavage also increases its durability. Its crystals are barrel-shaped prisms of the trigonal system.

2. The second hardest natural mineral known to science.

Its two varieties are sure to be on any list of gemstones. The red variety of corundum is known as ruby and all the other colors of corundum are known as sapphire.

Besides being used as gemstones, it is also utilized as an abrasive.