Any of the nearly 4,000 kinds of minerals could be cut into a gemstone—if a crystal of sufficient size and quality were found. But only about 15 different minerals are commonly used for gemstones. They combine desirable colors or optical effects with hardness and availability.

• How Does a Crystal Become a Gem?
• How are Gems Measured?
• What Makes a Gem Valuable?
• Where Do Gems Come From?