opal

This rock is a piece of volcanic breccia where reddish opal formed in gas pockets. Photo by James St. John. Licensed under CC By 2.0.

An unusual granular hyalite opal. Erongo Mountain, Usakos and Omaruru Districts, Erongo Region, Namibia. © Rob Lavinsky, www.iRocks.com.…

One of the rarest opal types, black opals from Australia. © Joel E. Arem, PhD, FGA. Used with permission.

Common and precious opal replaced this bivalve’s original shell, making it a spectacular fossil. Opalized fossil bivalve (Bulldog Shale,…

Like plastic opal imitations, Slocum Stone can have a variety of different appearances. Photo by Roy Goldberg. Public Domain.

While most Sterling opals show black potch lines, some lapidaries cut cabochons without these features. © Opal Auctions. Used with permission.…

The bright, saturated colors and columns visible in the bottom right corner indicate synthetic opal. © Opal Auctions. Used with permission.…

Finding a seam of precious opal is the goal of many opal prospectors. Photo by Géry Parent. Licensed under CC By-ND 2.0.

While the miner who found this nodule hit the jackpot, it’s entirely possible that there was no other precious opal nearby. Photo by James St.…