Perhaps one of the most mystique-laden gemstones, Opal is not technically a mineral because it does not possess a crystal structure. This beautiful opaque stone is composed instead of tiny silica spheres arranged in a pattern that refracts light into spectral colors, causing a unique tonal quality that can make Opal appear many different colors—or even colorless—depending on the cut, the lighting and the angle of viewing. We call this gem phenomenon “play of color,” and Opal is a master of it. Opal comes in many varieties, with Black Opal generally considered the most rare and desirable. Opal is often polished into a cabochon—a gem without facets—but occasionally it is faceted like other gems. October birthdays boast Opal (as well as Tourmaline) as their birthstone.