Blue Opal Sphere (with stand)Opal is composed of hydrated silica. It is an amorphous silicate, meaning it does not form crystals. Opals are structurally composed of tiny spheres of silica which, when arranged in perfect geometry, create the famous play of color associated with the stones. The symmetrical arrangement of silica balls acts like a prism, which allows the rainbow spectrum of colors to be visible to the naked eye.