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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 cement 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 graze 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.

Boulder Aesthetics

Because boulder opal is mostly ironstone matrix, it is priced by the piece rather than the carat. When selecting boulder opal, dealers advise jewelers to evaluate pieces on the basis of three critera.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.