| 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'
It 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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