What are rubies made of

Rubies are made of the mineral variety corundum, which is composed by aluminium oxide.

It's chemical formula is  Al₂O₃ 

There is a great diversity of colors according to the process of cooling down and purity of the stone.

Pure red rubies are normally the purest in corundum, while pink ones have been influenced by other isomers minerals surrounded, which intensify it's colors and richness composition.