What Is Opal

What is Opal?

Australia’s Hidden Treasure

– Foreword

Mysterious opals contain the wonders of the skies - sparking rainbows, fireworks, and lightning - shifting and moving in their depths.

Opal has been treasured throughout history around the world. Archaeologist Louis Leakey found six thousand year old opal artefacts in a cave in Kenya!

Roman historian Pliny described the beauty of opal as the combination of the beauty of all other gems: "There is in them a softer fire than the ruby, there is the brilliant purple of the amethyst, and the sea green of the emerald - all shining together in incredible union. Some by their splendour rival the colours of the painters, others the flame of burning sulphur or of fire quickened by oil." Opal was much loved and valued highly by the Romans, who called it opalus. At the same time, opal was also sought in what would become the Americas. The Aztecs mined opal in South and Central America.

Opal was also treasured in the Middle Ages and was called ophthalmios, or eye stone, due to a widespread belief that it was beneficial to eyesight. Blonde women wore opal necklaces to protect their hair from losing its colour. Some thought the opal's effect on sight could render the wearer invisible. They were recommended for thieves!

Opals are set in the crown jewels of France. Napoleon gave Josephine a beautiful opal with brilliant red flashes called "The burning of Troy," making her his Helen. Shakespeare found in the opal a symbol of shifting inconstancy, likening play of

colour to play of mind in one of the most apt uses of gemstone symbolism in literature. In Twelfth Night, he writes: "Now the melancholy God protect thee, and the tailor make thy garments of changeable taffeta, for thy mind is opal."

In the nineteenth century, opal was considered unlucky due to the plot of a popular Sir Walter Scott novel of the time. The heroine of the novel has her life force caught in the beautiful opal she wears in her hair and she dies when the fire in the opal is extinguished. However, Queen Victoria loved opals and often gave them as wedding presents. She and her daughters

created a fashion for wearing opal. Queen Victoria was one of the first to appreciate opals from an exciting new source:

Australia.

The story of opal in Australia begins more than 100 million years ago, when the deserts of central Australia were a great

Inland Sea, with silica-laden sediment deposited around its shoreline. After the sea receded and disappeared to become the great Artesian basin, weathering 30 million years ago released a lot of the silica into a solution which filled cracks in the rocks, layers in clay, and even some fossils. Some of this silica became precious opal. Opal is one of the few gemstones that are sedimentary in origin. Opal still contains 6 to 10 percent water, a remnant of that ancient sea. Gold panners in Australia found the first few pieces of precious opal in 1863. Mines at White Cliffs began producing in 1890.

A large percentage of the world’s stable commercial opal comes from Australia, and there are three distinctive regions where opal comes from. These are Lightning Ridge (the home of black opal) the South Australian fields Coober Pedy, Mintabie, Lambina and Andamooka (the main sources of light base opal) and the Queensland fields, where boulder opal originates.

The major opal producing fields for black opal is Lightning Ridge in New South Wales, Australia, and to a much lesser

degree Mintabie in South Australia. The most sought after black opal is red or multicolour on black, which is very difficult to find. Such stones are always cut on the fields and sell immediately.

Boulder opal is opal that has formed in a brown ironstone nodule, and is distinctive because it typically presents as a thin layer of opal over a dark brown base when cut. The opal can be quite superb.

Opal from Lightning Ridge is often considered to be the best and brightest in the world.

This booklet is meant as a guide, therefore we have tried to make it as non – technical as possible to increase the enjoyment to the opal enthusiast.

What is Opal?

Opal is a non-crystalline form of the mineral silica, which, despite its amorphous structure, displays an amazing degree of internal organisation. Opal is related to its more commonly found but highly crystalline cousins quartz and agate, and is formed from amorphous "balls" or lumps" of silica rather that from ordered, naturally faceted crystals.

The chemical composition of opal is SiO2H2O, silicon dioxide combined with water (an opal stone may contain up to 30% water.) The silicate minerals in the stone add to its weight, giving it a specific gravity ranging from 1.98 to 2.5 times that of pure water. Opal's scratch hardness is measured at 6.0 to 6.5 on the Mohs' scale, similar in hardness to quartz, a little more than halfway between the hardness of talc and diamond.

Most opal is more than 60 million years old and generally dates back to the Cretaceous period when dinosaurs roamed the earth.

It is found near the earth's surface in areas where ancient geothermal hot springs once flowed. The minerals bubbled up from beneath the surface of the earth and slowly, over the centuries, lined the walls of cracks, vents and underground cavities in the bedrock. Most opal is found where geothermal hot springs dried up during seasonal periods of rainfall and extended dry periods.

More than 90% of the world's quality gem opals come from Southern Australia, although it can be found in other parts of the world such as Brazil, Mexico, Czechoslovakia and Nevada. All black opal comes exclusively from Australia.

The story of opal in Australia begins more than 70 million years ago when the deserts of central Australia were an

gemstones that are sedimentary in origin. The water in opal is a remnant of that ancient sea.

The most striking quality of opal is its ability to refract and reflect specific wavelengths of light. In fact, the term "opalescence" was coined to describe this phenomenon. The size and spacing of the amorphous spheres of silica within the stone refracts specific wavelengths of light; each sphere refracting a single, pure spectral colour much like the

individual microscopic droplets of water in a rainbow. The interplay of these pure wavelengths of light gives opal its unique visual appeal, and makes it one of the most sought-after gemstones in the world.

Birthstone Months with Opal

April: Mystical Birthstone

October: Modern and Ayurvedic Birthstone

Aquarius (Jan 20-Feb 18): Birthstone

Libra (Sep 23-Oct 22): Birthstone/Lucky Charms

Anniversary Gemstone for Opal

Opal is the Anniversary gemstone for the 14th and 18th years of marriage.

Opal Mystical Properties

Opal is used to see possibilities.

Science has told us this about opal:

CHEMISTRY: SiO₂ . nH₂O. Water usually 6 to 10% in precious opal, can be as high as 21%

CRYSTALLOGRAPHY: Amorphous

REFRACTIVE INDEX: 1.44 - 1.47

HARDNESS: 5.5 - 6.5

SPECIFIC GRAVITY: 1.99 - 2.25

CLEAVAGE: None

HEAT SENSITIVE: Varies.

Opal is derived from the Latin word Upala that relates to precious stones and was considered the cupid paederos or child as beautiful as love.

Roman scholars described opal as having the green colour of emerald the brilliance of amethyst and that opal represented hope and purity which was later adopted by modern Asian countries who consider the opal as an anchor of hope.

The opal that the Romans referred to were from the present day country hungry and was crystal formed volcanic gemstones in the Carpathian mountains with flashes colour that was not seen any where else in the ancient world and really impressed the Romans

Ancient Greeks believed opal had the power to have sixth sense in foresight and also prosperity.

Cleopatra is said to have worn and opal to attract Mark Anthony.

Native Australian called aborigines’ have long history going back thousands of years and they considered opal to have spiritual properties and have many myths and legends about Opal.

They believe opal holds signs of their past ancestors and is sign of their presence and also a spirit named Muda who changes from man to pelican so pelican birds are considered spiritual also.

Asian countries had experienced opals from Indonesia and appreciate the colour spectrum of opals compared to gemstones such as ruby and sapphire which are mined in asia.

The opal has been considered as a gemstone of hope and security .

Japan buy opals as show of wealth and standing in the community as prefer perfect formed gem quality even if opal is small.

Opal is one of the worlds most spectacular gemstone and fortunately for those born in October it is their Birthstone.

This is under the Modern Birthstone chart.

For the traditional Birthstone chart Tourmaline is considered the birthstone.

For the Mystical birthstone chart Jasper is considered the birthstone.

And for Ayurvedic Birthstone chart Opal is also considered the Birthstone.

The Ayurvedic Birthstone list is from Ayurvedic Indian Medicine - a type of medicine used for over a thousand years

Now days we use the Modern birthstone chart so Opal is now strongly associated with October birthdays.

Opal is also considered the mystical birthstone for April.

Mystical Birthstone chart is the Tibetan thousand year old version and opal in those days are more closely associated with quartz gemstones that present day opals.

Opal is considered as the gem Of the Gods

And has mystical significance and is said to help in physician vision and to open our spiritual centres.

Opal is also believed to heighten weak emotions and to strengthen memory.

The word evolved from the Greek word "opallus" meaning to see a change in colour. Later the latin word "opalus" came to mean precious stone.

Opals stand in a class by itself. More than any other gem, each opal is distinctly an individual. No other stone has as rich and varied folklore. Opals are so unique they have their own descriptive vocabulary. They are the most delicate gems that are commonly worn.

Opal's fire was long thought to be the result of iridescence. However, with the advent of scanning electron microscopes, we now know that it is a result of diffraction.

Opal is an amorphous form of silica, (SiO2.nH2O) chemically similar to quartz, (SiO2) but containing 3% to 21% water within the mineral structure. Gem grade opals are usually 6% to 10% water content.

Opal is a sedimentary stone. Under proper conditions, water percolates through the earth, becoming rich in dissolved silicates. When it enters a cavity, the silicates are deposited as tiny spheres. If they are uniform in size and shape, they will diffract light. If they are random in shape and arrangement, we have common opal.

Volcanic ash gives black opal its color, but inclusions have nothing to do with the play of color. That is due entirely to the tiny spheres. They must be smaller than 1500 angstroms for blue and violet colors, but no larger than 3500 angstroms to produce oranges and reds. To put that in perspective, 20,000 spheres are about the size of the period at the end of this sentence.

Opal grows by filling in cavities, regardless of their shape. Hence, we have many pseudomorphs, materials with shapes that are unrelated to the chemical content. The most common are opalized wood and seashells.

source: http://www.gemsociety.org

Opal History

During the Cretaceous period (65-140 million years ago) deserts were an inland sea teeming with life. The sea gradually receded laying the foundation of silica that would form the opal of today. During the mid-tertiary period, changes in the earth’s climate caused quantities of soluble silica to be released from the sediment, finding its way down through cracks and faults, eventually hardening over time. This hardening continued to form common and precious opal.

The vivid array of colour in the opal is from the uniform alignment of the silica particles. The size of the particles determines the colour as light refracts through it.Unlike many other gemstones, opal doesn’t occur in lengthy veins or concentrations. Instead, small clusters may be spread over a vast area and divided into precious or gem quality and common. Opal is found in many varieties, but precious opal represents only a small percentage of the total mined.

It is estimated that between 95 and 99% of the worlds most precious opal is mined in Australia. Several unique regions of Australia specialise in different types of opal, from Coober Pedy's white and crystal opal in South Australia, through to the home of black opal, Lightning Ridge in New South Wales, up to the infamous Hayricks and Hammond's fields in northern Queensland for the boulder variety.

AUSTRALIAN SEDIMENTARY OPAL
– WHY IS AUSTRALIA UNIQUE?

David Horton, Managing Director, Opal Horizon Limited

View Full Geographical Paper (PDF FORMAT - requires Adobe Reader)

ABSTRACT

Australia currently produces about 95 per cent of the world’s precious opal from widely scattered fields throughout central Australia. No other country on Earth has such an abundance of this rare precious gemstone.

The sedimentary opal deposits of central Australia occur along generally flat-lying horizontallayers within 30 metres of the earth’s surface. They are a product of a unique set of geological events which occurred over a 100 million year period. These events can be summarised as follows:

1. Between about 122 million years ago (Ma) and 91 Ma, central Australia was covered by a vast shallow epicontinental sea. The sedimentary rocks which were deposited in this sea were derived from volcanic rocks and were organic-rich. These formed the principal host rocks for opal deposits in central Australia.

2. Following surface exposure through lowering of the sea level, these host rocks were subject to a prolonged sub-tropical weathering regime until about 40 Ma. Central Australia probably looked not unlike today’s Amazon Basin. During this time, the water table was close to the surface and was acidic releasing silica and iron from weathering of the host rocks.

3. The climate became more arid from about 40 Ma and, as a result, water table levels gradually lowered and the groundwater became alkaline. Mild tectonism at 24 Ma gave rise to subtle extremely long wavelength surface folds which facilitated both lateral and vertical migration under arid conditions of the earlier-released silica. Opal was preserved in the weathered profiles beneath the crests of the developing surface folds as water tables here lowered more rapidly due to tectonic uplift. Siliceous cap rocks discouraged erosion.

4. Over the last 10 million years, dissection and scarp erosion exposed the weathering profiles containing the opal.

Geologists believe that the volume of gems that have been produced over the past 150 years in Australia is but a minute fraction of the amount yet to be discovered.