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Opal
Information
Almost
ninety-five per cent of all fine opals come from the dry and remote
outback deserts of Australia.
The
group of fine Opals includes quite a number of wonderful gemstones,
which share one characteristic: they shine and sparkle in a
continually changing play of colours full of fantasy, which experts
describe as “opalising”. Depending on the kind, place of occurrence,
and colour of the main body, we differentiate Dark or Black Opal,
White or Light Opal, Milk or Crystal Opal, Boulder Opal, Opal
Matrix, Yowah Nuts from Queensland – the so-called “picture stones“,
and also Mexican and
Fire Opal. Opal variations are practically unlimited. They all
show in their own special way that unique play of colours – except
for Fire Opal, which due to its transparency, however, is
nevertheless also considered a Fine Opal specimen. If Opals are
lacking the typical play of colours, they are simply named “Common
Opal"
Opal’s colour
play emanates a very special attraction and fascination. But what
causes this phenomenon? This question was impossible to answer for a
very long time. Only when in the 1960s a team of Australian
scientists analysed Opals with an electron microscope, it was
discovered that small spheres from silica gel caused interference
and refraction manifestations, which are responsible for the
fantastic play of colours. The spheres, which are arranged in more
or less compact structures, succeed in dissecting the light on its
passage through the gemstone and turning it into all the colours of
the rainbow, always new and always different.
Australia is
the classical Opal country and today is the worldwide most important
supplier of Fine Opals. Almost 95 per cent of all Opals come from
Australian mines. The remaining five per cent are mined in Mexico,
and in Brazil’s north, also in the US states of Idaho and Nevada,
but recently the stones have also been found in Ethiopia and in the
West African country of Mali.
The history of Australian Opal began actually
millions of years ago, when parts of Australia were covered by a
vast inland sea, and stone sediment was deposited along its
shoreline. When the water masses flooded back, they flushed water
containing silica into the resulting cavities and niches in the
sedimentary rocks, and also the remains of plants and animals were
deposited there. Slowly the silica stone transformed into Opal, for
basically Opals are simply a combination of silica and water. Or, to
be more precise: Opals are a gel from silica, with varying
percentages of water.
In order to best bring out the play
of colour in a Fine Opal, the stones are cut and polished to round
or oval cabochons, or any other softly domed shape , depending on
the raw material. Only the best qualities of Fire Opal, however, are
suited to faceting. The Opal cutter will first of all carefully
remove any impurities using a diamond cutting wheel, before working
out the rough basic shape. The comes the fine cutting, the finishing
with sandpaper and then the final polishing with a wet leather
wheel.
Opal is often found as flat lenses, or thin
layers, bigger pieces are rather rare. If you leave a thin but
supporting layer of the harder mother rock, you will receive a
pre-stage of the Opal-doublets which are frequently used today for
mass produced jewellery. These are gemstone combinations consisting
of a surface from millimetre-thin Opal plates, which have been
mounted on Onyx, Obsidian, artificial black glass, or Potch-Opal.
Triplets have been developed from this design, here the Opal layer
receives an additional cover from Rock Crystal, Plastic, Hard Glass
or Lead Glass for protection.
Due to the
differing percentage of water, Opals may easily become brittle. They
always contain water – usually between 2 and 6 per cent, but
sometimes even more. Thus if stored too dry or exposed to heat over
a longer period of time, Opals will show fissures and the play of
colour will become paler. Therefore, Opal jewellery should be worn
as often as possible, for then the gemstone will receive the needed
humidity from the air and from the skin of its wearer.
Opals are not very hard: they only achieve
5.5 to 6 on the Mohs’ scale. Therefore they appreciate a protective
setting. In earlier days Opal’s sensitive surface was often oiled,
but today also sealing them with colourless artificial resin has
become quite popular.
The fiery, warm
glow of the fire opal beguiles jewel enthusiasts the world over.
This radiant orange simply cannot be overlooked! The fire opal,
which, like the other opals, consists of silicic acid with a
relatively high water content, was named for its colour. The colour
itself comes from fine traces of iron oxide. With a hardness of
between 6 and 6.5 on the Mohs scale, however, the fire opal is among
the somewhat sensitive gemstones which require a protective setting,
especially when worn as a ring stone.
Not all fire opals are the same. We
differentiate between the common fire opals, which, depending on
their quality, are either faceted or cut into cabochons, and the
especially valuable ones, which, in addition to their vivacious
colour, also have the gaudy play of colour typical of opals. But
with or without play of colour, the fire opal plays its part as a
top quality gemstone to perfection.
| RI: 1.44-1.47 |
SG:
1.98-2.20 |
Hardness:
5.5-6.5 |
Toughness: Poor
to fair |
| Treatments:
Often impregnated with oil, wax plastic or smoke
to improve play of colour |
| Care Tips:
Avoid ultrasonic cleaners, strong light and heat |
| Value: From
$10 to $15000 USD per carat - huge fluctuations due to
colour and type(Newman, 2003) |
Birthstone for October and 14th Wedding
Anniversary gemstone.
Opals can be securely purchased on this
web site at bargain prices ... see
here
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