Opal is a wonderful stone for earrings, pendants, brooches and rings. However, it is softer than most other gemstones. Opal has a hardness of about 5.5 to 6.0 on the Mohs hardness scale. Because of that opal is best suited for use in earrings, brooches and other pieces of jewelry that rarely encounter scuffs and impacts. When used in a ring, the best designs have a bezel that fully protects the stone – instead of being placed in a prong setting that exposes the stone to impact and abrasion.
Play-of-Color and Opalescence
Opal is a very common material that is found throughout the world. Most of this opal is “common opal” or “potch” which has a milky or pearly luster known as “opalescence.” However, rare specimens of opal produce brilliant color flashes when turned in the light. These color flashes are known as a “play-of-color.” Opal specimens that exhibit a play-of-color are known as “precious opal.” If the play-of-color is of high quality and large enough to cut, the material can be used to produce valuable gemstones.
A play-of-color in opal can be observed under three situations: 1) when the stone is moved, 2) when the light source is moved, or, 3) when the angle of observation is changed. The video on this page (above) illustrates the beautiful “play-of-color” in an Ethiopian Welo opal.
What Causes “Play-of-Color”?
Areas within an opal that produce a play-of-color are made up of microscopic spheres of silica arranged in an orderly network. This network of spheres acts as a diffraction grating. As the light passes through it is diffracted into the colors of the spectrum. The size of the spheres and their geometric packing determine the color and quality of diffracted light.