RUBY, sometimes referred to as oriental ruby, is a relatively rare and costly precious stone, generally red to deep crimson in color, but sometimes with pale rose or purple tints. Like sapphire (q.v.) it is a variety of corundum (aluminum oxide) which, next to diamond, is the hardest known naturally occurring mineral. The red color of ruby is thought to be due to the presence of traces of chromium. Rubies occur in crystalline limestones and in gem-bearing gravels derived from these rocks, the best gems coming from Burma.