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Opal Facts

opal , a mineral consisting of poorly crystalline to amorphous silica, SiO2•nH2O; the water content is quite variable but usually ranges from 3% to 10%. Common opal is usually colorless or white, but it may be gray, brown, yellow, or red; the color is due to fine-grained impurities. Opal is formed at low temperatures from silica-bearing waters and can occur in fissures and cavities of any rock type. Precious, or gem, opal has a rich iridescence and remarkable play of changing colors, usually in red, green, and blue. This is the result of a specific internal structure consisting of regularly packed uniform spheres of amorphous silica a few tenths of a micron in diameter; sphere diameter and refractive index determine the range of colors displayed.

The greater part of the world's supply of precious opal comes from the Coober Pedy and Andamooka fields in South Australia. The original source, known in Roman times, was in what is now E Slovakia. Precious opal has also been mined in Honduras, Mexico, and the Virgin Valley in Nevada. Fire opal is a bright red transparent or translucent opal that may or may not show a play of color.

How Opal Produces the 'Play of Colors'

orderly silica moleculesIt took the development of the electron microscope to work this out. Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal. 

Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect. 

Deflection & Diffraction 

Depending on the size of the spheres, varying colors of the spectrum are diffracted. So it is a combination of deflection (bending) and diffraction (breaking up) of light rays that creates the color in opal. If you move the stone, light hits the spheres from different angles and bring about a change in color. The name opal actually means "to see a change in color." The way in which colors change within a particular stone as it is rotated and tilted is called the stone's play of color.

How color is defined.

The size of the spheres has a bearing on the color produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the color will be.


Glossary of Terms:

Amorphous
Shapeless. Not consisting of crystals. Non crystalline. Glass is amorphous. Sugar is crystalline.Deflection
The bending of rays of light from a straight line.

Diffraction
The Breaking up of a ray of light into either a series of light and dark bands, or into colored bands of the spectrum.

Diffuse
To spread out so as to cover a larger space or surface. To scatter.

Fluorescent
A light produced by the electrical stimulation of a gas or vapor. Fluorescent lights have a similar effect on opal as a bright cloudy day--they do not properly bring out the colors in opal

Hydrate
A compound produced when certain substances chemically combine with water.

Incandescent
Glowing with heat (red or white hot) as in a light bulb which glows white hot, but produces a light that more closely simulates natural sunlight. Sunlight and incandescent lights bring out the natural colors in opal.

Opal
Opal comes from the Latin word opalus which means to see a change in color. Chemically, opal is hydrated silica, similar to quartz.

Opalescence
A play of color, similar to that of an opal.

Opaque
Not allowing light to pass through. The opposite of transparent.

Play of Color
The way in which colors change as an opal is tilted in different directions.

Silica
(Silicon Dioxide) A hard, white or colorless substance, that in the form of quartz, enters into the composition of many rocks and is contained in sponges and certain plants. The needle in the mouth of a female mosquito is made of silica. Flint, sand, chalcedony, and opal are examples of silica in different forms.

Spectrum
The band of colors formed when a beam of white light passes through a prism or by some other means (e.g. mist or spray, in the case of a rainbow) The full range of spectrum colors are: red, orange, yellow, green blue, indigo, and violet.

Sphere
A round three dimensional geometric shape whose surface is equally distant at all points from the center point.

Translucent
Letting light through without being transparent.

Transparent

___ Easily seen through. (glass like)

Resources:

Some opal facts are reproduced from www.opalmine.com. Check out their online encyclopedia for lots more great opal info.

Thanks to www.infoplease.com a wonderful online encyclopedia on all subjects.


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