Most people think of the Big Four ... Highlights: Gem-quality red beryl is so rare that rubies are 8,000 times more common, and gem-quality diamonds, 150,000 times more common.

Chalcedonies include many types of cryptocrystalline quartz gems and feature a number of ... and precious minerals such as emeralds—are the most common class of minerals, as well as the major ...

Compared to common opal, which is usually one color, black opal exhibits many different colors contrasted by a dark body tone. Frederick: It's simply the most stunning gemstone on the planet.