Summer 2007

Opal with Unusual Structure

Opal is one of the most fascinating of the phenomenal gems. Not only is it well known for its myriad play-of-color patterns, but it is also prized as a fossilizing agent and preservative of the structural features of other materials it replaces or envelops. In the past, GIA Laboratory staff members have seen opalized clams and other mollusks, portions of dinosaur bones preserved by opal, and opalized wood and bark showing excellent cell detail (see, e.g., the Fall 2001 G&G Lab Notes section, pp. 218–219).

The GIA Laboratory in Carlsbad recently had the opportunity to examine a 1.09 ct translucent light brown, cushion-shaped tabular cabochon that appeared to have a columnar structure of relatively evenly spaced cells, so that it resembled synthetic opal without the aid of magnification. The client was told that it came from an opal field in Queensland, Australia, but the structure caused some suspicion about its origin.

Standard gemological testing easily identified the cabochon as natural opal. With magnification, it appeared that the unusual structural pattern was generated when the opalizing solution was deposited over a drusy surface covered with numerous quartz microcrystals, as shown in figure 1. The blue and green play-of-color displayed by the opal highlighted this structure like a colorful mist, adding a mysterious, alien haziness to the gem (figure 2) that seemed to float over the surface as the stone was moved. Throughout the years, the GIA Laboratory has had the opportunity to identify many marvelous opals from a variety of localities worldwide, but this is the first time such a structure has been encountered.

John I. Koivula
GIA Gem Laboratory
Carlsbad, California