There are only four precious stones: ruby, sapphire, emerald, and diamond. All the others stones of gem quality are qualified as semi-precious stones.
The price that a ruby will fetch is determined by color. The brightest and most valuable “red” is termed pigeon blood-red, and are much more valuable that rubies of similar quality. After color follows clarity: similar to diamonds, a clear stone will be more valuable, but a ruby without any needle-like rutile inclusions may indicate that the stone has been treated to enhance the color. Finally, the cut of the stone and its size (measured in carats) also determines price.
Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems only moissanite and diamond are harder, with diamond at the top of the list.
Mining
The main source of rubies was the Mogok Valley in Upper Myanmar (Burma). That region has produced some of the finest rubies ever mined, but in recent years very few good rubies have been found there. The new source is in central Myanmar. The area of Mong Hsu began producing rubies during the 1990s and is now the world’s main ruby mining area.
Rubies of lesser quality are found in Thailand, Cambodia, Tanzania, Madagascar, Vietnam, Nepal, Tajikistan, and Pakistan. A few rubies have been found in the U.S. states of Montana, North Carolina, and South Carolina. More recently, large ruby deposits have been found under the receding ice shelf of Greenland. Rubies were found in the Waseges River area of Kenya in 2002.
Spinel, another red gemstone, is sometimes found along with rubies in the same gem gravel or marble, and is even mistaken fo ruby on occasion.