Around the world the Opal is famed for a fantastic – and unique – ability: the diffraction of light known in the industry as opal’s ‘play of colour’, or its milky opalescence.
The distinctive ‘play of colour’ attribute is caused by interference, diffraction and refraction of light by minor spheres of silica gel. When light passes into the gem, it bends around the ‘chips’ of silicon, the tiny ‘spheres’ of silica and oxygen to create an whole spectrum of colours within one stone.
Opals have another defining characteristic; they have a relative softness when compared to other gemstones, rating 5.5 to 6.5 in terms of hardness. This is due to the fact opals always contain at least 3 percent water – and in some cases can reach 30 per cent.
COLOUR, CLARITY & CUT
This gem can be found in many colours. These include an almost translucent white, a milky white, oft yellows, pale reds, browns, greys and blacks.
As previously mentioned, diffraction can create flashes of another colour within the opal. Green, blue and yellow ,are most common flashes, while orange, red and violet are quite rare. Opalescence and adularescence (best described as a milky quality that comes from deep within the stone) are usually soft blue in colour
There are three main subgroups of opals: common opal (also called potch), precious opal and fire Opal
Common opal is typically and does not host the distinctive play of colour – instead bestowing viewers with an ‘opalescence’ effect. This type of opal is often mixed with agate opal or moss opal or other gemstones .
Precious opal exhibits the extraordinary ability to diffract light into the famous rainbow of colours that change when the observation point is altered – the aforementioned ‘play of colour’.
Fire opals can display the play of colour found in precious opals, but are notable for the deep orange or red body colours. Nearly all gems within the opal family will display few internal fractures or inclusions
These gems also tend to hold a wax-like or slightly resinous to vitreous lustre which lends itself to either the stone’s opalescence or play of colour
They are normally cut en cabochon – a shape which (when combined with high or low domes) maximizes the play of colour or adularescence.
OPAL GEMSTONE PROPERTIES:
|Chemical Formula:||SiO2_nH2O – Hydrous silicon dioxide|
|Crystal Structure:||Amorphous; kidney or grape-shaped aggregates|
|Color:||All colors, partial play of color|
|Hardness:||5.5 to 6.5 on the Mohs scale|
|Refractive Index:||1.37 to 1.52/td>|
|Density:||1.98 to 2.50|
|Transparency:||Transparent to opaque|
|Double Refraction or Birefringence:||None|
|Luster:||Waxy to resinous|
|Fluorescence:||White opal: white, bluish, brownish, greenish; fire opal: greenish to brown|
An opal’s value is determined predominantly by the body colour saturation (or transparency) or the , unique characteristics of its play of colour or opalescence
Within the transparency characteristic, it is worth noting translucent specimens usually fetch higher prices than opaque.
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ORIGINS & IMITATIONS
Nearly 95% of white precious opals were mined from Australia. Deposits of note include White Cliffs and Lightning Ridge in New South Wales, and Andamooka and Coober Pedy in South Australia.
Yet other locations around the world have bestowed miners with Opals – such as Brazil, Honduras, Japan, Russia and the USA.
Lastly, synthetic opals do exist, and a few gemstones can exhibit iridescent effects reminiscent of the stone.
NAME, FAME & LEGEND
Opal probably derives its name from the word for ‘stone’ in Sanskrit.
It is told in Australian Aboriginal legend that when the Creator came to earth to bestow his message of peace to humanity, opals were created wherever his feet touched the ground.
Later the famous Roman author Pliny the Elder, cited opals as stones which amasses the most fantastic characterises of the world’s most brilliant gems; The Almandine’s Fine Sparkle, Amethyst’s Deep Purple Tone & Sapphire’s Rich Blue Captured Together In One “Beautiful Combination”.
Throughout time, peoples have alleged that Opals have healing properties and are able to solve depression and true love for its owner.
Of the world’s opals, it is arguably ‘Halley’s Comet’ which is the most famous – and has earned its place in the Guinness World Records books as the largest uncut black opal in the world.