Opals That Display A Play Of Color Are Known As Precious Opals. When An Opal Shows No Play Of Color, How Is It Described?

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When the structure of Opal was examined. It was discovered that it was made up of small spheres of silica surrounding a central nucleus. The difference between common and precious stones was that common opals had a random structure with spheres of different sizes, whereas the precious opals had regular mound of identical sized spheres.

The "play of color" refers to the parade of colors that can be seen inside of an opal. This color is produced by the diffraction of light as it travels through the silica and the areas between the spheres and also the refraction of light from the surface of the gem. The angle at which the stone is viewed can change the color it displays.

The unevenness in the common opals, combined with some smaller sized silica spheres, means that the light cannot be diffracted in the way that it is in precious opals. The color displayed by precious opals is partially dependent on the size of the spheres it is composed of. Violet color is displayed by the smallest spheres and red by largest.

Black opals are the most valuable of precious opals as the darker background of the stone makes the play of color the most prominent. Crystal opals are precious opals with a lighter background. The precious opal set in fire stone is a boulder opal.