What is Opal?

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.

Resources
© Stuart Bazga Article Source:
http://EzineArticles.com/

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.

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