Via Flickr:
A large specimen of natural opal on a dark matrix stone, this image was captured through the glass of a display case. Opals contain alot of water, anywhere up to 20%. Rings featuring a large opal gem are delicate and must not be exposed to temperature extremes or accidents that can crack them. Opals are often displayed in water so the iridescent glow called “play of color” appears. Not all opals have the color flash. This piece measures about two inches, a very large specimen.
For more information, this wikipedia article is good:
en.wikipedia.org/wiki/Opal