Gemstones & Pearls/Gemology
Some see their opals as half empty, others as half full
Opals are among the most kaleidoscopic and varied gemstones. They can exhibit play of color or not; they can be opaque, translucent or transparent; they can resemble colorless glass or a bowl of orange jelly. Their identities can fool even seasoned pros.
This Mexican opal resembles a glass of milk half transparent and colorless, half translucent and white. It comes from the personal collection of gem dealer Pete Flusser of Overland Gems, Los Angeles, CA. What makes this gem unique, aside from its remarkable composition, says Flusser, is the cleverly faceting that shows all of opals distinct characteristics in one gem.
The unusual formation in this opal may be attributable to flow structures, which are described in The Photoatlas of Inclusions in Gemstones by E.J. Gübelin and J.I. Koivula (ABC Edition, Zurich, Switzerland). The book describes opal as amorphous meaning it has no crystal form or structure. Opals are composed of a hydrous silica gel formed by individually crystalline, submicroscopic spheres of closely packed cristobalite and trydimite. A dense cloud or concentration of the white cristobalite may account for the milk seen in this opal from Queretaro, Mexico.
by Robert Weldon, G.G.