Meet the Gemstones: Outback Opals – Boulder, Koroit, and Andamooka Opal
With colors that seem to appear magically from the depths of the stone, one of the most fascinating gemstones, without question, has to be opal. Even opal-holics, though, know that opals with a white body color can leave something to be desired, even when the play of color is strong. If the same flashes of color are seen against a dark background, they are much more dramatic.
ABOVE: This steel and gold pendant by Bette Barnett makes an arresting frame for the blaze of color in this dark ironstone matrix of this Koroit opal. Photo courtesy Bette Barnett/Sparks Gallery.
“Boulder opal,” such as that coming from the Koroit mining field in southwestern Queensland, is found naturally in a dark ironstone matrix. When possible, opal cabs are cut so that the matrix forms a backing that strengthens the often thin veins of play-of-color material and intensifies the phenomenon.
But often, the opal is so enmeshed in the matrix that it isn’t possible to cut out the opal without tremendous loss of beautiful material. This rough can be cut not only to take best advantage of the distribution of play of color, but to incorporate matrix into the face of the stone, creating fascinating patterns and patches of color. For this reason, boulder opals, such as Koroit, are especially intriguing to designers. Each stone is unique. By mimicking the opal patterns in the lines and shapes of the metal work, designers can create unforgettable one-of-a-kind pieces of jewelry.
Koroit opal doesn’t tend to crack or craze. The ironstone of the Koroit area also tends to be less “sandy” than matrix from other areas, with less tendency to undercut during the fashioning process. As a result, better quality material is not stabilized or treated in any way, although lower quality material may have the pits or sandy areas of matrix sealed. Be sure to ask.
Boulder opals usually have areas and patches of intense color. But sometimes the color is spread out throughout the matrix. What then? That brings us to treated Andamooka opal.
Opal is a hydrothermal mineral which means it forms when hot, supersaturated solutions, usually under pressure, seep into fissures in rocks and cool and/or undergo a reduction in pressure. The minerals in the solution precipitate out into cavities or seams. In the case of Andamooka opal, the opal often precipitates into the small pores in limestone.
Pinpricks of Light
Although the color of these opals may be striking and bright, and may be spread out in a handsome blaze, the spots of color are disseminated in the limestone like pinpricks of light, not exhibited in broad flashes as in common opal. In addition, the body color is white, making the spots of color more difficult to appreciate. To make these stones more marketable, they are treated to give them a dark background.
Opal Treatment: Sugar Water
A common folk treatment for the improvement of color in an opal is to soak the opal in sugar water or just water. Some unknown person, possibly a miner, took this a step further. He discovered that if he first cooked Andamooka opal and limestone matrix in a saturated solution of sugar water, then followed that by a bath in a solution of sulfuric acid, the sugar would carbonize, turning it dark in colors from gray to brown to black. The play of color in the opal then shows up brilliantly against a dark background, the way a rainbow shows up against dark storm clouds.
Because gemstones are individuals—and opals more individualistic than most—not every opal will treat the same way. Inclusions of harder or softer material in the limestone as well as the concentration of opal can affect how well the treatment takes.
(Note: Though there are instructions on sugar treating your own Andamooka rough out on the Internet, sulfuric acid is extremely dangerous. Lapidary Journal Jewelry Artist does not recommend anyone try this.)
Sugar Treatment Care
In the best cases, the sugar carbonizes to a dark gray black and the spots of opal show up brilliantly. Some say these stones rival the finest Lightning Ridge black opal. They can be a beautiful center stone in a piece of jewelry and a less expensive alternative to black opal. However, because the treatment is only a millimeter or so deep, these stones are usually best set into jewelry, such as necklaces, brooches, and earrings, that will not receive hard wear. The shallow treatment also makes it inadvisable to try to recut a stone to fit a mounting. If you are buying these stones, be sure you understand the care they require. If you are selling them to a customer in a piece of jewelry, be sure to pass those care instructions on.
Sharon Elaine Thompson is a GG and FGA who has been writing about gemstones and jewelry for Lapidary Journal Jewelry Artist since 1987. She also writes a line of birthstone romance novels under the name Liz Hartley.