How is opal formed?
Precious opal is made up of tiny uniform spheres of silica which were deposited in sedimentary and volcanic areas during the cretaceous period, about 110 million years ago. The silica hardened and formed spheres which fit together into an orderly three dimensional frame, sitting in a bath of silica solution. It is the orderliness of the spheres which separate precious opal from common opal.
How are the colors created?
Light passes through the transparent spheres in a direct line, but when it hits the silica bath, it is bent and deflected at different angles producing a rainbow effect. Depending on the size of the spheres, varying colors of the spectrum are diffracted. It is the combination of deflection (bending) and diffraction (breaking up) of light rays that create the colors in opal. The word opal actually means change in color. The way in which the colors change within a particular stone as it is rotated is called the play of color.
The size of the spheres determines the colors seen. Larger spheres produce the oranges and reds, while the smaller ones produce blues. The other colors are seen with spheres of medium size throughout the range. The more uniform the arrangement, the more brilliant and intense the color.
How is the value determined?
The value of opals differs greatly depending on the body tone, play of color, the colors present, brilliance, pattern, and size. Black opals from Lightning Ridge, Australia are the most highly prized, with the darker body tones being more valuable than the lighter.