About Opal

Opal is a hydrated amorphous form of silica SiO2·nH2O);  its water content may range from 3 to 21% by weight, but is usually  between 6 and 10%. Because of its amorphous character, it is classed as a  mineraloid,  unlike crystalline forms of silica, which are classed as minerals. Spencer opal has a very low water content making it very stable in most cases. 

 There basically two kinds of opal: precious and common. Precious opal displays play-of-color, common opal does not. The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which  it formed, opal may be transparent, translucent or opaque and the background color may be white, black or nearly any color of the visual spectrum. Black opal with a red play of color is considered to be the rarest, whereas white, gray and green are the most common. Spencer opal offers something extremely unique, Pink based opal. I believe that Spencer is the only source for pink based opal with color play on earth.

Ultraviolet Flourescence.  Some Spencer opals will glow under long and short wave ultraviolet lights. I have several very nice "glowers". Mostly greens and reds. Amazing how different they can look under long wave vs short wave. Spencer opal harbors some rare earth elements that fluoresce wonderfully, and as the name says some are quite rare. Many years ago a Fluorescent Mineral Society use to visit the mine as a group to search for those elusive rare earth glowers.

Opal is found in many parts of the world. Australia, Ethiopia,Mexico,the modern day Czech Republic, Hungary and of course the USA and Spencer Idaho. They are not all the same. Mexican opal for example has for the most part and orange base with or with a play of color and is mostly translucent. Spencer opal forms perfectly flat in very thin layers the vast majority of the time making it the best opal in the world for triplets. 

Opal has a long history, some recorded, some not. The first recorded name was offered by the Romans. They called it "Opalus" which means precious stone. It was treasured by ancient Romans 200 years before Christ. The ancient Greeks called it "Opallos" which means "to see a color change". Maybe the most famous description opal was offered by Pliny the Elder a Roman philosopher........"From within this precious stone one can see the living fire of Ruby, the glorious purple of Amethyst and the sea green of Emerald all playing together in an incredible display of color and light." (several versions of this quote have been offered over the centuries)

The primary formation of the mine is a rhyolite and obsidian flow full  of gas pockets. The Opal Solution or silica, was a secondary deposit carried by geyser activity. As a result of several eruptions over a period of time, the opal lays in layers within the gas pockets.

 This Formation information is courtesy of The Spencer Opal mines