The 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 Australia.
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 OPAL
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.

Body Tone
The base tones of light, dark and black opal range from colourless, white, through the various shades of grey, to black.

Transparency
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 opal.

Light Opal

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.

Matrix Opal

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 formed.
Boulder matrix opal is found in Queensland and can be distinguished by the ironstone host rock.
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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