Opal, the birthstone for October, is a stunning gemstone known for its vibrant play-of-color. Australian opal is the most famous and valuable variety, accounting for over 90% of the world’s opal supply. The most prized types include black opal from Lightning Ridge and white opal from Coober Pedy. These opals display brilliant flashes of color, shifting with the light, thanks to their unique internal structure of silica spheres.