Micro-World Gems & Gemology, Spring 2021, Vol. 57, No. 1

Opal with Fluid Inclusion and Mobile Bubble

Gas bubble moves within a fluid-filled void in a Mexican opal.
A Mexican opal containing a mobile gas bubble in the lower edge of the void (left). Tilting the stone causes the bubble to change position within the cavity (right). Photomicrographs by Jessa Rizzo; field of view 3.57 mm.

A Mexican opal was recently examined at the GIA laboratory in Carlsbad. The inclusion shows a euhedral void with a mobile gas bubble (see above). The void mimics the shape of a barite crystal that might have once been present in the opal. The barite crystal was likely dissolved during a dissolution phase during the growth of the opal, leaving behind only a fluid-filled void. Toward the end of the opal’s formation, the temperature decreased, causing the trapped fluid to separate into liquid and gas phases. Because of the small size of the bubble relative to the void, rocking and tilting causes it to move about. Fluid inclusions in opal are generally quite rare, and one with a mobile bubble is exceptional.

Jessa Rizzo is a staff gemologist at GIA in Carlsbad, California.