Opals are unique in that they contain all of the colors of other gemstones. It is the official birthstone of October.
Opals are a non-crystaline form of quarts and have up to 20% water mixed in. Opals are formed of grids of silica spheres of varying sizes (typically around 0.2 microns) which each refract light differently. This creates the beautiful rainbow of colors mixed throughout an Opal.
The Romans used to believe that the Opal was the most precious and powerful of all gemstones because it had all of the colors of other gemstones. Royalty often valued Opals for its protective powers and were often set in crowns and royal jewelry to ward off evil. A ground up powdered version was used once as protection from nightmares.
Mineral Group: Hydrated Silica (Non-crystalline Quartz)
Colors: All Colors
Refractive Index: 1.37-1.47
Specific Gravity: 2.15 (+0.08, -0.90)
Mohs Hardness: 5 to 6.5