Opal is one of the most fascinating and popular gemstones due to it unsurpassed rich play of color. It also has an interesting and unique structure – it is not truly a mineral; but rather, a mineraloid. It is amorphous (without a defined crystalline shape), thus lacking one of the scientifically accepted standards defining a mineral: a mineral must have a crystal structure. However, all is well since it was given a pass and allowed to join the true minerals team! Opal is also hydrated, with a typical water content between 6 and 10%. Due to its “play of color” (a phenomenon in which different colors are displayed when viewed from different angles or light sources), all variations of the light spectrum are possible depending on the variety and specimen. Base colors include colorless, white, yellow, red, orange, green, brown, black, blue and pink. (Opal has almost as many names as locations (ok, probably more). Some are IMA approved, others are trade names, and others are local nicknames. I have chosen to stick with Opal as the overarching category and allow each individual specimen to reflect its unique characteristic name and variety.)
*Common Opal (without play of colors) is very prevalent and occurs worldwide, while Precious Opal (with play of colors and gem quality) is found in few localities, including Australia, Ethiopia, Mexico and the USA (Nevada).