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.
Most opal is cut and polished to form a cabochon. “Solid” opal refers to polished stones consisting of precious opal. Thin opals may be combined with other materials to form attractive gems. An opal triplet has a third layer, a cap of clear quartz on the top that acts as a protective layer for the opal. The top layer also has a magnifying effect, to emphasize the play of colors of the opal beneath.
Besides the gemstone varieties showing play of colors, other kinds of common opal include the milk opal (milky bluish to greenish), resin opal (honey-yellow with a resinous luster), wood opal (replacement of the organic material in wood with opal), menilite (brown or grey), hyalite or Muller’s glass (colorless, glass clear), geyserite or siliceous sinter and diatomite or diatomaceous earth.
Fire opal is a transparent to translucent opal, with warm body colors of yellow to orange to red. Although it does not usually show any play of color, occasionally it exhibits bright green flashes.
Fire opals that do not show play of colors are called jelly opals. Other types are Cantera opal and Mexican water opal (colorless opal exhibiting either a bluish or golden internal sheen).
Girasol opal is a type of hyalite opal that exhibits a bluish glow. It is not a play of color as seen in precious opal, but rather an effect from microscopic inclusions.
Peruvian opal (blue opal) is a semiopaque to opaque blue-green stone. It does not display pleochroism.
Black Opal or Opal with a dark grey body shows the most brilliant play of colours. Crystal opal is more transparent with a deep play of colours. White or milky Opals show more diffuse colours and are the least expensive Opals.