The physical structure of opal is unique. Tiny spheres of silicon form a neatly stacked structure interspersed with water. The nature of opal's structure and the cause of its color was not understood until the advent of electron microscopes, which allowed scientists to discover the secrets within opal. When these spheres are consistent and orderly in their arrangement, play of color will result. When disordered, there is no play of color and common opal or potch is the result. Play of color is the result of interference of the light as it travels through these layers of tiny spheres.
Opals are typically classified depending on their body color, host material, and transparency. Body color is the background color – not the flashes of color. This can vary from colorless (crystal opal or jelly opal) to milky (white opal,) to shades of gray, brown or blue (dark opal or semi-black opal) to black (black opal.) The opaque to translucent areas with no color play are called potch. Potch can vary in color and is often what determines whether an opal is considered a black opal or a crystal opal. Fire opals usually have no play of color, but have a translucent to transparent body color ranging from cherry red to orange to light yellow to colorless. Some will display play of color (color flashes) as well. Fire opal is commonly faceted, but can be cabbed as well. Queensland boulder opal is opal with an ironstone (boulder) host rock that is still attached. Matrix opal (also known as “opal with matrix’) are any opals where the veins or grains of opal are interspersed within a host rock. Common opal is the same as potch in that it is opal, but without any play of color. Blue and pink opals from Peru are examples of common opal, as is the green prase opal from Tanzania. There are a number of rare varieties of opal for collectors to discover.
For the past one hundred years, most of the world’s play of color opal was sourced from a handful of prominent mining areas in Australia, namely Lightning Ridge, Coober Pedy, Andamooka and Mintabie. Mexican fire opals added a distinctly different option in opal, and Brazil produces both fire opal and play of color opal. About ten years ago, the Ethiopian opal was “rediscovered”, but the original finds produced opal that was largely unstable. The Wollo area is now producing a beautiful variety of stable crystal opal, much of which has a body color from colorless to yellow to orange to brown, even rarely in black. It also produces some fire opal in red orange and yellows.
While any play-of-color is welcome, opals displaying flashes of red with all colors are the most prized by collectors. Lightning Ridge black opals are the most valuable among all opals. Crystal opals are also highly prized, as are the purer red shades of fire opal, known as cherry opal. The patterns seen are infinite in their variety and make each opal unique.
Opals should be protected from chemicals, high heat, and harsh wear. Because opals contain 5 to 10 percent water, some opal has been known to dry out and craze (small hairline cracks) after a short while or even after a lifetime of use in a piece of jewelry. Ideally they should not be stored in dry, air-tight containers like safe-deposit boxes. Opal jewelry should be cleaned with mild soapy water and a soft brush or cloth.
Ethiopian opals require different care than other opals due to their ability to absorb water. While it has been popularly considered a good idea to soak opals in water, this does nothing for an opal and in fact, should be avoided in the case of Ethiopian (Wollo) opals. If they do get wet, they will lose their play of color, becoming almost opaque. Not to worry – if allowed to dry in a normal environment, they will revert to their original appearance, usually within an hour, but this depends on size of the opal and humidity. For this reason, it is best to avoid dirty or colored water and even oils. Clean them with a dry, soft cloth.