Opal is a gemstone consisting of hydrated amorphous silica with the chemical formula SiO2.nH2O.
Australia is by far the world's largest producer of opal, being responsible for approximately 95% of the world's total production. It is widely considered to be Australia's national gem.
Opal is formed naturally from silica gel over an extremely long period of time, through the effects of pressure and heat. The three main forms of precious opal are boulder opal, white opal and black opal
The rarest and most valuable opal is the black opal, and in its crystalline form it is amongst the most valuable gemstones in the world. Although known as black opal, this highly precious gemstone is famous for its striking colours. It can display a wide range of colours, from greens, purples and blues to yellows, oranges and reds. Each piece of opal is completely unique. It is the black crystal opal that we set in our inlay jewellery. It is all mined in Mintabie in South Australia, virtually right in the centre of the country.
The crystalline form of black opal has a relative hardness of 6.5, compared to other forms of opal at 5.5, so it is hardier than the other forms and more difficult to scratch. Set inlaid in 18ct Yellow Gold or Sterling Silver, these pieces are extremely durable and can be worn all of the time.
Opal is a precious stone composed of silica similar to quartz with a varying amount of water trapped within the mineral structure. 90% of all opal found is valueless "potch" (opal without colour, usually white or greyish white) but the remaining 10% is the brilliantly coloured and highly prized gem. Precious opal often contains 6-10% water.
Romans considered opal more precious than all other gemstones, believing that it held the colours, and therefore the combined beauty of all precious gems.
Opal.
How Opal Colour is produced
When the electron microscope was developed scientists worked out that Precious opal is made up of tiny uniform spheres of transparent hard silica, which fit together in an orderly three dimensional frame, sitting in a "bath" of silica solution. It is the orderliness of the spheres that separates precious opal from common opal.
Light passes through the transparent spheres in a direct line, but when it hits the 'bath' of silica, it is bent and deflected at different angles, thus producing a rainbow effect.
Deflection & Diffraction
Depending on the size of the spheres, varying colours of the spectrum are diffracted. It is a combination of deflection (bending) and diffraction (breaking up) of light rays that create the colour in opal. If you move the stone, light hits the spheres from different angles and bring about a change in colour. The word opal actually means "to see a change in colour." The way in which colours change within a particular stone as it is rotated and tilted is called the stone's play of colour.
How colour is defined
The size of the spheres has a bearing on the colour produced. Smaller spheres bring out the blues, from one end of the spectrum. Larger spheres produce the reds from the other end. The more uniform the spheres are placed, the more intense, brilliant and defined the colour will be.
How good is it anyway? and how much is it worth?
Opal is often said to be of the greatest 'value' to the person who chooses it. Each piece is different, each seems to have its own personality and its appeal is very personal. Each piece is special, and your piece is the most special of all. Some people like unusual opal specimens such as shells, or pipes (opalised tentacles of sea creatures). Others like only a particular colour or pattern. polished opals
Opal is classed by colour, brilliance, quality, clarity, variety and pattern of the colourful spheres
The main price determining factors are:
Solid Opals that are cut in a solid piece are known as solids. These are the most valuable.
A doublet opals are composed of two layers consisting of an opal top fabricated over opal matrix material, potch (common non-precious opal) basanite, jade, black onyx or obsidian. The value of these are lower than an solid not-assembled composite.
A triplet opal has three parts. A piece of precious opal in the centre, a clear top and a darkened base (usually potch or glass) to highlight the colour.
Fire Opal is orange or reddish opal from Mexico that has no play of color or iridescence.
Boulder Opal results from splitting of a seam of opal running the rock or boulder. The polished piece will have opal as its face and the host rock material as its base.
White Opal are white base, more common variety of opals that are usually the least expensive.
Black Opal is more rarer than white and warrant's a higher price. There are various shades of "black" opal. A "Black 3" is a true black color. True black opal is simply to die for.
Semi-Black Opal are opals with base colors from medium
gray to dark gray.
Gray Opals have a base color of medium to lighter gray color.
Crystal Opal is transparent enough to see through them. Various colors and fire can be seen as light rolls through them.
Cut Opal Calibrated and free form cut white, crystal, black opal suitable for ring, brooch, and other settings. Calibrated opals are stated specific sizes in mm. For example a 8 x 10 mm calibrated stone will fit in a purchased settings designed for a 8 x 10 mm stone.
Rough Opal Uncut material suitable for free-form or calibrated cutting, ideal for lapidary clubs, hobbyists, custom design work, or inlay work.
We do not grade our Opals as our pricing reflects the quality and value of each stone. Our prices are based on 30 years of experience in mining, cutting and selling opal. We carry only Australian Opals - Lambina, Lighting Ridge, Coober Pedy, Andamooka, and Mintabie. The following information is for your reference and assistance in selecting an opal for your taste and needs.
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