Opals are clam-shaped, fiery at NYC exhibit

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Opals are clam-shaped, fiery at NYC exhibit

By Teresa Novellino

March 10, 2009

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These opals formed in the fossils of clams that date back tens of millions of years.

New York--After receiving a series of luminescent donations--from Oregon fire opals to spectacular clamshell-shaped beauties from Australia's legendary Coober Pedy mines--the curators at the American Museum of Natural History recently realized they had a small opal exhibit on their hands.

The display, which opened at the Manhattan museum on Feb. 26, is limited to one special exhibit case, situated near the entrance of the museum's Harry Frank Guggenheim Hall of Minerals. While the museum does not put a price tag on the opals, getting a look at some of the more unusual ones is well worth the price of admission.

"Black opals are the most prized, but with all gems, it's a question of taste," said George Harlow, the museum's curator for earth and planetary sciences, during a recent unveiling of the exhibit for the press.

About 85 percent of the world's gem-quality opals are mined in two states in Australia: Queensland and New South Wales. But they have also been found in places that some jewelers might not know about, or expect, including Oregon and Ethiopia.

Perhaps the most unusual items in the exhibit are a set of three precious opals that formed inside fossilized clams and look something like baked clams, stuffed with white, luminescent opal. They were discovered in Coober Pedy mines in South Australia, with two of them gifted to the museum by Michael Lamb, and a third donated by Mabel Lamb.

How did these unusually shaped gems form inside fossils? Since clams were alive as far back as 110 million years ago, during the Cretaceous Period, it is estimated that the clam fossils date back to tens of millions of years ago, Harlow said.

Long after the shells were buried underneath sediment, the climate changed and the Great Artesian Basin of Australia turned into a desert environment. Seasonal rains weathered the feldspars in the sediments and released silica into solution, which traveled down to the levels of the buried clamshells. Prior solutions probably dissolved the shells leaving a cavity or mold, and the silica-rich fluid filled the cavity with silica gel, which transformed into opal, Harlow said.

For other examples of fossilized opals, click here.

In one case, the entire opal had been polished, probably because it didn't retain clam features. The other two are polished on one side and showing clam structure on the other, Harlow said.

Several of the donations for the exhibit come from members of the jewelry industry. A quartet of spectacular-looking stones--including one black opal doublet and two triplets, all from Australian mines, as well as a cabochon from Honduras--were gifted by William Larson (of Pala Gems) and Jeanne Larson. Also included in the exhibit are Ethiopian opals in a range of fiery browns and reds, gifted by Evan Yurman of David Yurman.

A small but fiery dice-shaped opal included in the exhibit is from Yowah, Queensland, and was gifted by Bill Kasso.

To see more opal images, visit our 10X blog.

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