Wednesday, September 16, 2009

Opal

When someone mentions opal most of us think of the white material from Australia that is colored with sparkles of rainbow that change as the stone is rotated. It’s almost magical. But the science behind this phenomenon is simple.
As light passes through the surface of an opal, it goes between the larger silicate spheres (atom sized) within the opal. The larger silica spheres act as a prism breaking up the light into its rainbow of colors. The spheres of silica must be between 0.15 and 0.30 microns (a micron is one ten thousandth of a millimeter). These spheres of silica must also be arranged in a chaotic order (amorphous), meaning they can not be arranged in neat little rows.

Terms to know when talking about opals…
Reflection: this describes the light that bounces off the surface of an opal.
Refraction: this describes what happens to light as it hits the surface of an object and goes through it. It can happen every time the light reaches a new surface.
Dispersion: this is when light is broken up into its colors as a result of reflection or refraction.
Diffraction: when light is split up into its colors by passing trough a thin slit or slot like the silica spheres of opal.
Potch: common opal with no play of colors.
Jelly Opal: usually has no play of color buy the opal is a vivid color such as orange, red, yellow, or green.
Fire Opal: jelly opal from Mexico with a play of colors. A term erroneously used to describe any opal with fire.
Precious Opal or Common Opal: another name for potch opal without color.
Hyalite Opal: a type of opal that is water-clear with no play of color, usually fluorescent.
Opel: the name of an old car that didn’t last long on the market.
Triplet: the layering of a thin sheet of opal between a clear top and a black backing.

(Knowing these terms can also help you choose a quality opal over one that is "mismarketed")




U.S. Opal Locations…
Utah has an opal location near Milford. It is often called Bacon Opal, Bubble Opal, Lee’s Opal, Candy Stripe Opal, and Milford Opal. It has no play of colors as it is hyalite opal. Even though it is opal, this type has very little commercial value. Some pieces when cut just right will show “bb” size bubbles with a little refraction of color but this doesn’t compare to even poor opals.

There are a number of locations for good opal in the US. Louisiana has a fine sand where each grain is a single is a colorful opal and rarely some solid pieces are found that can be cut and polished. In northern Idaho is Spencer Opal. This opal forms in paper thin layers of puddles of opal that are deposited in lava. Virgin Valley in Nevada is famous for the opal replacing wood. This is arguable the best opal in the world for color. Unfortunately, it is unstable making it useless for jewelry you expect to last.

Of coarse, when there is money involved, there is also fraud. There is much synthetic and imitation opal and some of it is so good it is very difficult to detect.


Museum Display of the Month:
We have changed the geode display to highlight “Opals from Around the World”. Some are for sale and some are just to oogle at. Come in and check them out.

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