Opal is the National Gemstone of Australia. It is a hydrated amorphous form of silica with a structure that makes it diffract light and take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Of these hues, the red against black are the rarest, whereas white and green are the most common. It varies from opaque to semitransparent.
Precious opal shows a variable interplay of internal colors whereas Common opal, called “potch”, does not have this quality. The term “opalescence” is used to describe this unique and beautiful phenomenon, play of colors.