Opal (Precious) Classification
Virtually Any Color
Precious Opal, Opal With Play-Of-Color
Key tests are play-of-color, refractive index and fluorescence. Opals might phosphoresce, and this is a strong indication of its natural origins, as synthetic or simulant opals do not phosphoresce.
Some natural opal is polymer impregnated to make it more stable or improve its appearance. Sugar treated opal will have an RI between 1.440 to 1.450 and a SG between 1.98 to 2.05 both of which are lower than untreated opal. Smoke treated opal typically has an RI between 1.380 to 1.390 and a lower SG than untreated opal. If there is an absence of play-of-color, opal is often referred to as "common opal" instead of "precious opal. "Black crystal opal is transparent to semitransparent shows strong play-of-color with a dark bodycolor.Black opal is translucent to opaque with play-of-color against a dark background.Contra-luz opal is a transparent opal that only displays play-of-color in transmitted light. Crystal opal is transparent to semitransparent with a clear bodycolor and strong play-of-color. Jelly or water opal has no color and is transparent to semitransparent and displays weak or no play-of-color.Noble or precious opal is another name for stones that display play of color. Semiblack or gray is translucent to opaque with a light to medium gray background and displays play-of-color. White opal or milky is translucent to semitranslucent on a white background and displays play-of-color.