During the 1980s, when the Japanese pushed black opal prices to levels beyond the reach of nearly everyone but themselves, far-less-expensive boulder opal came on strong in America as a substitute. Then, during the withering world recession of 1991-92, when the Japanese also balked at the high cost of black opal, this alternate variety came on even stronger in Japan.
In both countries, there was no other choice—other than assembled stones such as opal doublets, or lab-grown such as Gilson opal. Detractors of boulder opal don’t care much that this gem is the only all-natural option for those who want the look of black opal without the price. Says one, “Settling for boulder opal instead of black opal is like settling for caviar made from cod, in place of sturgeon roe.”
Not! Boulder opal isn’t some form of low-grade black opal but a separate opal variety unto itself. Like the black opal it often resembles, it is found only in Australia. In fact, the first discovery and full-scale mining of Australian opal, in 1872 and 1878 respectively, involved boulder opal from Queensland, thus day the only producer. It wasn’t until 1903 that miners to found black opal at Lightning Ridge in the next-door state of New South Wales. Eventually this magnificent opal became the world’s choicest and supplanted demand for the boulder variety.
Ninety years later few appreciate the role boulder opal played in first establishing Australia as the world’s chief opal source. Now that there’s a revival of interest in boulder opal, it might be worthwhile to see why the gem earned Australia quick renown more than a century ago.
Wasteland Wonder
Boulder and black opal could be called wasteland wonders, since deposits of both are scattered throughout Australia’s vast stretches of semi-desert known as “outback.” Indeed, aridity is a precondition for creation of opal, as Peter Keller explains in his engrossing new book, “Gemstones and Their Origins.”
Simply put, opal is a silica gel deposited as a filling in rock fractures by seepage of seasonal rains into the ground. During the long dry spells that follow the rains, the gel hardens by evaporation of most but not all of the water content. Because this gel is composed of countless tiny stacked spheres, it diffracts light into distinctive prismatic color patterns—provided these silica spheres are the same size and arranged in orderly rows. The more precise the arrangement of these light-diffracting spheres, the better the color play. Of course, colors stand out better still when the host rock in which opal forms is dark rather than light.
It is here that boulder opal differs significantly from black opal. Found in boulders of charcoal-brown, iron-rich sandstone called “ironstone,” it is the only precious opal cut with host rock (known as “matrix”) as a backing. Although black opal is sometimes thought to be cut with matrix, this backing is, in fact, colorless opal called “potch.”
Because boulder opal forms in thin films reminiscent of the tops of opal doublets, many a novice mistakes them for such. (Doublets are made by fusing opal slivers to a black base of glass, onyx and other materials with cements that bubbles when touched with a hot point.) Indeed, for years boulder opal was sold as “nature’s doublet.”
The fact that boulder opal is never more than a thin layer on thick matrix works for more than against this gem. First, it does not craze and has superb durability. And, second, while it isn’t as a rule cut into cabochon or calibrated sizes (ruling out use in mass production jewelry), boulder opal lends itself to free-form shapes that are ideal for custom jewelry.
Belated Believers
Most U.S. importers of boulder opal are former detractors who became belated believers in it. That’s a blessing, considering what specialists must go through to find adequate amounts of it. There’s barely enough to support the burgeoning American market. John Manning, Western Opal Corp., Los Angeles, estimates that no more than 5% of Australia’s annual opal production is of the boulder variety. Since at least 70% of the gem’s output is cut and sold in Australia, mostly to Japanese tourists, Manning finds himself pitted against a handful of colleagues for the meager quantities of boulder opal that are exported.
And it isn’t just American dealers who must scramble for goods. Australian dealers find their home advantage counts for little when it comes to buying boulder opal because the country’s retailers constantly outbid them. In fact, it took a tourism-crippling Qantas airline strike in October 1989 to help those few Australian dealers who carry boulder opal in depth to replenish their inventories. “As soon as tourists stopped coming, retailers stopped buying,” says John Traurig of Jayson Traurig Bros, Pty. Ltd., an Australian opal firm with an office in Dallas. “For a couple of weeks, I had the market practically to myself. I bought every piece of boulder opal I could find.”
Boulder Opal Aesthetics
Because boulder opal is mostly ironstone matrix, it is priced by the piece rather than the carat. Although connoisseur specimens command $5,000 to $50,000 today, most pieces sold to American jewelers range from $100 to $600 in price. When selecting boulder opal, dealers advise jewelers to evaluate pieces on the basis of three criteria:
Ideally, stones should face up with a continuous sheet of opal. However, since such stones are exceedingly rare, collectors will sometimes allow for a tiny trace or two of surface ironstone—but only if it contributes to, not detracts from, its beauty.
Next, the stone should have intense color—the result of the hue-heightening contrast between its translucent face and its dark-brown background. As a general rule, the thinner the band of opal on the matrix, the richer the color. So while the opal layer should be more than a veneer, thickness past a certain point can be a drawback.
Last, color patterns should be broad, even and definite. Although boulder opals often have rich multi-colored combinations of red, yellow, blue and green, stones more commonly feature blue and green exclusively. At their best, these latter stones have large swirls of flashing color that many liken to an earthscape as seen from space.
Please note: this profile was originally published in 1992 in Modern Jeweler’s ‘Gem Profiles/2: The Second 60’, written by David Federman with photographs by Tino Hammid.
The 6.82-carat boulder opal shown in the header image is courtesy of Western Opal Corp., Los Angeles.
