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Opal
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General Information:
Opal is the birthstone for the month of October along with the gem tourmaline.
The name opal is derived from ancient sources: the Sanskrit Upala which
means "precious stone"; the Latin Opalus; and the Greek
Opallios which both mean"to see a color change." It is
believed that opal artifacts were first discovered in a cave in Kenya, these
opal were most likely from Ethiopia and mined around 4000 B.C. After the
discovery of opal it began being shaped into shapes for jewelry and various
decorative pieces, much like other precious gemstones. While opal is found in
more than twenty countries, since the late 1800's Australia has dominated opal
production. Many ancient leaders had a strong passion for this gem, one even
offered to trade one-third of his kingdom for a single opal.
Opal is hydrated silica (SiO2.nH2O). This process occurs when silica dissoves
into groundwater from surrounding strata. The permeable rock allows some of
this water to evaporate over millions of years. Silica has a natural tendancy
to repel water, so it would tend to settle together, after settling it forms
into a gel which continues to lose more water. The gel substance after loosing
enough water would cement together and form a hard mineral, often creating a
continuous substance. That is not always the case though, sometimes, there was
just enough of the substance that when it hardened there were gaps between the
different silica spheres. The gaps act as a diffraction grating, splitting
incident light into its full spectrum, which is what causes the vibrant
multi-hued color that is seen in precious opal.
There are four types of opal; boulder, crystal, white, and black. Boulder opal
is found in Australia, in cracks and crevices of brown ironstone boulders.
Boulder opal is very low in water, so the gem rarely cracks or crazes. Crystal
opal is pure opal and is transparent with vibrant flashes of color. White opal
is the most common type of opal, it has a creamy white appearance with flashes
of color. Black opal is primarily found in the Lightening Ridge mines in
Australia. Black opal is the most desired and valuable, with breathtaking color
flashes against the dark base color. |
Origin:
Opal can be found in over twenty countries including: Australia, Zambia,
Ethiopia, Guatemala, Poland, Peru, Hungary, Canada, New Zealand, Indonesia, the
USA, Brazil, and Mexico. |
Common Treatments and Lab Created
Forms:
Since opal is made of 2-6% water, when left in a very dry climate it can easily
crack or craze. Sometimes opal is oiled to prevent this from happening. Opal is
also occasionally coated with a protective resin. Opal cannot be heated, it
will cause it to crack.
There is a created form of opal, but it does not contain water, making it not
have the identical composition of natural opal. Since it has no water however,
it is much harder, and has the same flashes of natural opal. |
Physical Properties:
Moh's Scale Hardness: 5.5-6
Specific Gravity: 2.0-2.5
Fracture: Conchoidal
Colors: Variety of colors, with a rainbow of flash.
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