Blue opal gemstone is one of the many varieties of opal stone that is available in shades of blue-green and blue. Sometimes, the term “blue opal” also describes the opaque varieties of opal found in Machu Pichu, which generally possesses a teal color. Unlike other gemstones, blue opal is not a mineral – rather, it is classified as a mineraloid. What it means is that the mesmerizing blue opal is composed of spheres of microcrystalline silica that scatter blue light through a phenomenon known as the play of colors.