Opal is a hydrated form of Silica. Opals water content can be as high as 21% by weight which can make it stress fracture as it dries. The water Content is usually between 6% and 10%. Because of it's amorphous character, it is classed as a mineraloid, Crystalline forms of silica on the other hand are classed as minerals. The internal structure of precious opal makes it diffract light. Light that looks like little flashes of fire. Opal ranges from Opaque to transparent and the flashes of color come in all the colors of the rainbow.