Ruby has been the world's most valued gemstone for thousands of years. Ruby was said to be the most precious of the twelve stones God created when he created all things and this "lord of gems" was placed on Aaron's neck by God's command. The bible says that wisdom is "more precious than rubies," that is to say very valuable indeed. In the ancient language of Sanskrit, ruby is called ratnaraj, or "king of precious stones" and ratnanayaka, "leader of precious stones."
In fact, rubies are today still more valuable and rare than even the top quality colorless diamonds. A 16 carat ruby sold at auction for US$227,301 per carat at Sotheby's in 1988. A 27.37 carat Burmese ruby ring sold for US$4 million at Sotheby's in Geneva in May 1995, or $146,145 per carat. A 32 carat ruby sold for US$144,000 per carat at Sotheby's in 1989. In contrast, eight D-color internally flawless diamonds over 50 carats were sold in the past 9 years and the largest, a pear-shape of 102 carats, fetched a mere US$125,000 per carat. Top rubies are so rare even the world's top gem dealers must incessantly comb estate sales and auctions to find them. Sizes above five carats are particularly rare.
Ruby is the gem quality of the mineral corundum, one of the most durable minerals which exists, a crystalline form of aluminium oxide. Corundum has a hardness of 9 on the Mohs scale and is also extremely tough. In its common form, it is even used as an abrasive.