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Boulder Opal

During the 1980s,when the super-strong yen pushed black opal prices to levels beyond the reach of the rest of the world, far-less-expensive boulder opal came on strong in America as a substitute. Then, during the withering of world recession of the early nineties, when the Japanese also balked at the high cost of black opal, this alternative 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-growns such as Gilson opal. Detractors of boulder opal don’t care 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 in place of black opal is like settling for caviar made from cod, instead 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, to this day the only producer. It wasn’t until 1903 that miners 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.

Gems of the Outback

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 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 these 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 patters—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 sand 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.”

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Boulder Opal