The gemstone
Opal is the official October birthstone as adopted by the
American National Association of Jewelers in 1912. It is also
the birth stone for the Zodiac sign of Libra. Opal is suggested
as a gem to give on the 13th wedding anniversary. Opal is 5.5 -
6 on the Mohs scale of hardness. Most opal is 50-65 million
years old, dating back to the Cretaceous period when dinosaurs
roamed the earth. Opal has been mined for centuries, at least
since Roman times when they extracted the Opal from areas now
within the Czech Republic. The Aztecs made use of local Mexican
sources as did the Spaniards when they exported the material
back to Europe. Today most precious Opal comes from Australia
with significant sources from Mexico and the Western United
States. Australia is famous for its white and black precious
Opal. Black Opal comes from the Lightning Ridge region of
Northern New South Wales.
It is the most rare and most valuable opal. Fine quality black
Opals often cost more per carat than Diamonds. The term Black
Opal is misleading because the Opal is not actually black, but
instead has a very dark base. This dark base enhances the
brilliant colors know as "fire". Fire Opal, the bright red,
reddish-yellow, orange body colored Opal is the second most
important Opal commercially. Until recently, the best Fire Opal
came from Mexico. Boulder Opal occurs in the boulder opal belt,
an area between the New South Wales border and northwest
Queensland. Boulder opal forms on a dark ironstone base (the
host rock) and occurs as a thin uneven layer adhering to the
ironstone. Because of the uneven layers, sometimes part of the
ironstone is visible on the surface of the stone. Queensland
Boulder Opal accounts for a small percent of the world market,
and because of its brilliance and rarity can bring up to $1000 a
carat.
Click here for a picture in natural form.
Click here or
here for more polished and natural specimens.
Description:
Opal has over one hundred variety and trade names, but the list
of accepted or commonly used names is much shorter. The most
important and most widely known Opal is the precious Opal.
Precious Opal may be subdivided further by color modifiers,
white, black, pinks, and blue, which describe the body color of
the Opal. When the Opal is mixed through the ironstone it is
called matrix Opal. Unlike other Opals which can be brittle,
heat sensitive, break and scratch easily (some varieties
self-destruct through the loss of water), Boulder Opal is very
durable due to the ironstone backing the Opal forms on. Because
water content within the Opal is very low it almost never cracks
or crazes as it ages. Opal's name evolved from the Roman word "opalus"
from the Greek word "opallios" meaning "to see a change of
color". The Greek word was a modification of the ancient Indian
Sanskrit name for Opal, "upala", which meant "precious stone".
If one spoke in mixed tongues, then Opal would be "opallios
upala", "to see a change of color precious stone".
Chemistry:
Opal's chemical formula is SiO2 - nH2O,
Hydrated Silica. Opal forms as silica from decomposing rocks
mixed with ground water which creates a silica gel that collects
and hardens in underground cavities and fissures. Opal is
considered a mineraloid because this structure is not truly
crystalline. The chemistry of Opal is primarily SiO2
and varying amounts of water. The amount of water varies from 5
- 10% and greater. This water can help geologists determine the
temperature of the host rock at the time the Opal formed.
Although there is no crystal structure, (meaning a regular
arrangement of atoms) Opal does possess a structure. Random
chains of silicon and oxygen are packed into extraordinarily
tiny spheres. These spheres in most Opals are irregular in size
and inconsistent in concentration. Yet in Precious Opal, the
variety used most often in jewelry, there are many organized
pockets of the spheres. These pockets contain spheres of
approximately equal size and have a regular concentration, or
structure, of the spheres. This has the effect of diffracting
light at various wavelengths, creating colors. Each pocket
produces a different color, with a different intensity depending
on the angle from which a viewer sees it. There are two distinct
types of Opal, common and precious. The way the silica particles
form determines which type. In precious Opal, the silica
particles are packed in regular rows and layers. Moving the
stone causes light to diffract, or split, as it grazes the Opal
surface. This light diffusion shows iridescent flashes of green,
blue, aqua and sometimes yellowish or red colors which are
referred to as "fire".
Boulder Opals are graded into specific groups which have been
defined by the Australian Gemological Association:
Boulder black opal
Boulder crystal opal
Boulder light opal
Boulder matrix opal
Yowah nuts |
Boulder splits
Seam and vein opal
Sandstone opal
Pipe opal
Wood opal |
Legends, Myths and Healing
Properties:
 As
indicated by the derivation of its name, Opal has centuries of
history as a treasured gemstone. Historically, beliefs
associated with the wearing of Opal have varied. The early
Greeks thought that Opals gave their owners the powers of
foresight and prophecy. Romans adored it as a token of hope and
purity. Eastern people regarded it as sacred. Arabs believed it
fell from heaven. In the nineteenth century, superstitions grew
about the bad luck or fate that could befall one for wearing
Opal if it were not the wearer's birthstone. Today, these
superstitions have diminished, but some people still believe it
is bad luck to wear Opals. Other legendary properties include:
aids inner beauty, faithfulness, and eyesight and helps recall
past lives. |
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