Opal is a colorful mineral of hydrated form of Silica related to Quartz. Its internal structure makes it diffract light, taking on many colors depending on the conditions when it was formed. Those colors can range from clear through white, red, orange, yellow, green, blue, purple, rose, pink, brown and black. Opal varies in optical density from opaque to semi-transparent. Opal is the national gemstone of Australia, a producer of over 93% of the world’s supply.