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Opal Time

About Opal

About Opal

There are 3 main types of natural Australian Opal, White Opal, Black Opal and Boulder Opal.

White Opal (or light Opal) is the name given to the 'milky' opal that has good colour play coming from within the opal. (N5-N9 grading scale) White opals are distinguished by their pale white or light body tone. As with any kind of opal, white opals can display any colour of the spectrum in a beautiful play of colour. Because of their pale body tone, white opals generally have less vibrant colour than boulder opals and black opals. White Opals are mined in the South Australian opal fields of Coober Pedy, Andamooka, Mintabie and Lambina, and also Lightning Ridge in NSW. Shop in our secure online store for White Opals.

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Black Opal (or dark Opal) is the name given to the opal that is dark in body tone colour with beautiful opal colours oozing out from it. (N1-N4 grading scale) Black Opal is the highest value opal, as it is by far the rarest form of Australian precious opal. Black Opal will have varying degrees of darkness, with the darkest being the most valuable. Black Opal is only found in one location, just to the south of Queensland opal fields, in and around Lightning Ridge in NSW. Shop in our secure online store for Black Opals.

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Boulder Opal is the name given to opal that has formed inside ironstone boulders of rock. These rocks or boulders are round or of spherical formation and have veins of opal running through them. The opal forms inside the boulder in cracks or fissures throughout the host rock. Boulder Opal can be veins sitting atop the host rock or thinner veins running throughout the rock like a matrix, Boulder Matrix Opal. When good opal is found, the colours can be amazing, rivalling that of top quality Black Opal. Boulder opal is only found in the middle of Queensland Australia, it spans from near Cunnamulla to around Winton in the north. Shop in our secure online store for Boulder Opals.

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Crystal Opal is the name given to opal that is crystal clear when held up to the light. In bigger pieces this can produce beautiful colour when worn on the body, however smaller pieces must be inlayed into jewellery to create the vibrant colours. We have a great range of inlay jewellery, hand made by Harri, available in store or on this site. Shop in our secure online store for Crystal Opals.

Colours in Opal

As opal is refracting light, the colour in an opal can vary immensly, in fact opal can have every colour we know of. It can have colours varying from blue to green, and purple to the more valuable red, orange and yellow. The colours your opal displays plays an important part in the value of the opal. Interesting patterns also increase the value of an Opal.

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Opal is made up of silica sphere clusters and the size and arrangement of these spheres determines the colours and flash of the opal. The smaller silica shperes produce the blue colours and the larger silica spheres produce the orange and red colours. Interstingly the size of the wavelength of the light must match the size of the silica sphere in order to produce that particulaur colour in opal. As opal has an amorphous structure of silica it produces varying colours in every single stone.

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What is OPAL, How did it form, Why the colours???

Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica, and carries this silica-rich solution into cracks and voids (natural faults or decomposing fossils). Once the water evaporates and settled under gravity in a void, it forms layers of silica spheres. This cycle repeats over very long periods of time, and eventually opal is formed. The solution is believed to have a rate of deposition of approximately one centimetre thickness in five million years. The origin of the silica is still unknown as silica is a very common substance and yet precious opal is so rare. There must be something special about the opal silica. There are theories which suggest it may be from a kaolin clay, rich in silica, and is an organic form of silica from rotting feathers or bones and other organic matter.

Precious opal is composed of tiny silica spheres. When the spheres are stacked like chains in a three-dimensional grid and the spheres are as big as a wavelength of visible light, then colours will be visible. The exact formation of these silica structures is a hotly debated subject, and scientists, geologists and miners themselves, all have unique theories. One of these theories suggests that the opal was once a kaolin clay (rich in silica) and has been deposited into the cracks and fissures in the ground. As the water in this kaolin dried out, the opal formation began. When there is a fault in the ground, friction occurs, and this we believe causes a static electric charge. A combination of the silica rich kaolin and a very slight electrical charge has grown the amorphous structure of silica that we know of as precious opal.

RESEARCH has uncovered why precious opals can only be found in Australia's red centre - or potentially on Mars.

Australia produces 95 per cent of the world's opals, but scientists and geologists have never been able to explain how this rare gemstone has formed. "We did not know its origin, why it forms at such shallow depths or why it can be found in central Australia and almost nowhere else on earth," University of Sydney associate Professor Patrice Rey said. He said the drying out of Australia's central landscape 100 million years ago created conditions that trapped silica-rich gel in rock, which later solidified to form opals.

As the Great Artesian Basin’s inland sea regressed an "extraordinary episode of acidic weathering" took place, caused by the exposed pyrite minerals, releasing sulphuric acid. As the surface cracked, silica-rich gel became trapped in the veins in the rock, which was later covered and protected from the weathering by secondary minerals, allowing the silica to solidify to form opals.
 
Central Australia is believed to be the only place on earth where acidic weathering of that scale took place. The only other area where similar conditions have been observed is on the surface of Mars, Professor Rey said. Non-precious opal deposits were discovered on the red planet by NASA in 2008. "If you look at Mars and the red centre, they share similar characteristics," Professor Rey said. "They went through the same weathering process, so potentially precious opals might exist there." 
 

Please visit our other information pages about opal

Valuing Opal

Where does opal come from?

How to cut and polish Opal

How to mine for Opal

Opal posters in the shop

Solids, Doublets and Triplets

Is Opal a bad luck stone?

When was opal first discovered?

Other interesting facts about opal