National Gemstone of Australia
Ninety-five percent of the world's precious opal comes from
Australia, where unique geological conditions allowed the formation of this rare gemstone.
Most light opal is found in South Australia, black opal in New South Wales, especially
Lightning Ridge, and boulder opal in Queensland.
Opal is Unique, in one precious opal you may see all the colours of the rainbow. As early
as the 1st century A.D. the Roman Pliny wrote:
"For in them you shall see the living fire of ruby, the
glorious purple of the amethyst, the sea-green of the emerald, all glittering together in
an incredible mixture of light."
No other gemstone can display all the colours of the spectrum in
such an infinite variety of shades, patterns and brilliance. No two are identical; for
this reason the proud owner can delight in the fact that their gem is unique.
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Across the world precious opal occurs in
very few locations because it required a very special series of geological and climate
phenomena to coincide for opal to form.
These special criteria occurred in what is now the great desert regions of central
Australia, which produce 95% of the world's precious opal.
During the Cretaceous period (65-140 million years ago) this area was an Inland Sea. Fine
marine sands rich in silica were deposited around the shoreline. The great Artesian Basin
formed when the sea receded.
During the mid-Tertiary period around 30 million years ago, deep weathering caused changes
to the sediments releasing large quantities of soluble silica. This solution traveled
along faults and joints in the ground infilling cracks and voids. The gel eventually
hardened to form common opal, and in rare circumstances it formed precious opal.
It was estimated that it takes 5 million years for an opal to form one centimeter thick!
On occasion, fossil bones and shells from the early Cretaceous period (120 million years)
have been replaced with opal, and in rare circumstances, with quite brilliant colors.
These make magnificent showpieces and are sought after by collectors and museums.
The major commercial outcrops in Australia are Lightning Ridge, Coober Pedy, Mintabie and
the Queensland fields. Opal also occurs in Mexico, Brazil, Honduras, Indonesia and western
United States of America.
Opal derived its name from
"Opalus" which meant "to see a change in color." Opals were
popularized by the Romans and this early opal is thought to have come from Cernowitz, a
mountainous region in Hungary, now Slovakia.
Mark Antony loved opal. Indeed, it is said that he so coveted an opal owned by Roman
Senator Nonius that Antony banished the senator after he refused the sell the almond sized
stone, reputed to be worth 2,000,000 sesterces (U.S. $80,000).
Precious opal has been used in the crowns of kings, and the crown jewels of France
included fine specimens of opal. Napoleon presented his Empress Josephine with The Burning
of Troy, a magnificent opal with brilliant red flashes. Shakespeare described the opal as
"that miracle and queen of gems."
Queen Victoria became a lover of opal and wore opals throughout her reign. Her friends and
her five daughters were all presented with fine opals. Opal became highly sought after
during the Victorian era because the Royal Court of Britain was regarded as the model for
fashion around the world, and fine quality opal had recently been discovered in far off
Today fine quality gem opals are owned and worn by many Royals and Heads of State.
The Australian Opal and Gem Industry
Association (AOGIA) has recently agreed on an official nomenclature for opal, which is
being widely distributed and adopted by international bodies in the industry. See the
AOGIA website http://aogia.polygon.net for
the full text of the nomenclature.
Natural opals are those which have not been treated or added to in any way by mankind,
other than by cutting and polishing. Natural opals are usually described as light,
dark/black, boulder,and matrix. Although boulder opal has an ironstone backing, it is
regarded as a solid natural opal because this backing occurs naturally. The variety of
natural opal is determined by the two characteristics of body tone and transparency.
The base tones of light, dark and black opal range from colourless, white, through the
various shades of grey, to black.
Opal of any body colour will be opaque, translucent or transparent. When it is transparent
or very translucent, and the colour clarity is sharp, it is often referred to as crystal
Natural opals with a base tone ranging from colourless to medium
grey are called light opal. Some people refer to these as "white," although this
expression should only be used where the body colour is very milky. Light opal makes up
the bulk of precious opal. It comes from all opal fields, but today the majority is found
in Coober Pedy, South Australia.
Black/dark opal shows a play of colour within or
on a dark body tone, while the play of colour of a black opal is within or on a black body
tone, when viewed from the face up. It can be crystal or opaque. Some black/dark opals
have a light crystal colour bar on dark opal potch (colourless opal), giving the otherwise
light opal a dark appearance. Even expensive black/dark opals may have only a very thin
colour bar on black potch.
Most black/dark opal is found in the mines around Lightning Ridge, NSW. Because of its
relative scarcity compared to light and even boulder opal, it tends to be more expensive,
given equivalent colours, clarity and patterns. Black/dark opal exhibiting bright flashes
of red is extremely rare.
Boulder is a variety of precious opal which has
an ironstone host rock forming naturally as part of the gem. Often just a thin vein of
precious opal is present. It occurs in specific locations over a wide area of Queensland
where the opal fills cracks or voids in ironstone boulders. Boulder opal can be black,
dark or light depending on the appearance of the stone when viewing the presentation face.
The term matrix opal is commonly used where the opal is intimately
diffused as infillings of pores or holes between grains of the host rock in which it was
Boulder matrix opal is found in Queensland and can be distinguished by the ironstone host
Andamooka matrix opal is a porous material from Andamooka, South Australia, which is often
treated to enhance the colour by depositing black carbon by chemical treatment in the pore
spaces in the stone.
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