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Australian Opals
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Australian Opal - Types
Black Opal
Black opal is characterized by a dark body tone causing brightness of colour which is unmatched by lighter opals. Black Opals are mined predominantly in Lightning Ridge, New South Wales, and are the most famous, and chased type of opal. The term ‘black opal’ does not mean that the stone is black in colour (a mistake made by many), it means the opal has a dark body tone in comparison to a white opal. The other common mistake is the “semi-black” opal. Do not be fooled, as an opal is either black toned or not. The semi-black is actually classified in the opal world as a grey opal, giving it the name of “semi-black” is just an advertising trick.
White Opal
Also known as ‘milky or opaque opal’, a white opal feature light white body tones, and is mined in a number of mining places in Australia, though it is predominantly in South Australia. White opal is more common and because of its body tone, generally does not show the colour as well as black opal. Nevertheless, white opals can still be absolutely magnificent in colour if a good quality stone is found.
Boulder Opal
Boulder opal forms on a natural ironstone boulders backing and comes solely from areas in Queensland. This type of opal is often cut with the ironstone left on the back, as the opal seam is usually quite thin. Leaving the ironstone on the back means that boulder opal can be very dark and beautiful in colour.
Crystal Opal
Crystal opal is any of the above types of opal, which has a transparent, or semi-transparent body tone (you can see through the opal). Crystal opal can have a dark or light body tone, leading to the terms “black crystal opal” and “white crystal opal”.
Fire Opal
The Fire opal is a term that evolved from within North American, which means any of the above kinds of opal (usually a black opal) that exhibits a considerable amount of red in the opal. With Red being the rarest of colours they are also one of the most valuable.
Matrix opal
Matrix opal is the opal that occurs as a network of veins or infilling of voids between grains of the host rock. This delicate matrix comprises precious opaline silica as an infilling of pore spaces in silty clay-stone or ironstone. A common and beautiful form of matrix opal is the Andamooka matrix opal. Andamooka matrix opal is often enhanced by soaking the rough opal specimen in a sugar solution and then boiling it in acid based solution to deposit carbon in the available pore spaces. This process causes the lighted background to ultimately go dark to black. The boulder opal matrix is another kind of matrix opal, found at Yowah in Queensland, which in its natural state consists of brown ironstone with small deposits of opal interspersed. This kind of opal is not treated.
Doublets & Triplets –The following diagrams demonstrate the differences between doublets, triplets, and solid opals;
Solid (cabochon) – Most cutters prefer to produce the opal as a solid cut ‘en cabochon’ if the gem is sufficiently thick. The opal is left in its natural state and simply shaped and polished on the cutter’s wheel.
Doublet –A thin veneer of opal may show enhanced colour with a dark backing of either black or grey silica material, or a thin slice of common opal cemented to the back of the opal with epoxy resin.
Triplet –A slice of quartz may be used to cap the thin opal veneer to protect it from abrasion. This produces a three-tiered gemstone known as a triplet, which can often display brilliant colours.
It is a cheaper method of presentation and can enhance the appearance of the opal.
Doublets and triplets are a good affordable substitute for solid black opals. Doublets pricing can range from $10 to $400 each and triplets $2 to $60 each. However their disadvantage is the susceptibility of the triplet (more so) and the doublet to water penetration between the layers, due to the fact that the layers are attached with glue. A doublet or triplet may become waterlogged if immersed in water repeatedly. If this happens, the stone will take on a ‘foggy’ appearance.
Brightness of Fire
Level: |
Name: |
Description: |
1 |
Faint |
Shows a play of color only under direct sunlight, and even then, the fire is faint or almost non-existent. |
2 |
Dull |
Shows some color under low light, but is dull even under indirect sunlight or the grading lamp. |
3 |
Bright |
Shows fair color under low light and very nice fire under indirect sunlight or the grading lamp. |
4 |
Very Bright |
Shows good color under low light and sharp crisp color under indirect sunlight or the grading lamp. |
5 |
Brilliant |
Shows exceptionally bright crisp color under indirect sunlight or the grading lamp, and often shows even brighter in subdued light. |
Valuing Opals |
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Valuing Opal is very difficult and the following key factors are considered the most relevant in any appraisal.
Body Tone/Color - Black or dark opal is considered more valuable than white or light opal, technically opals are rated according to their Body Tone " N " Value from 9 (black) down to 1 (light transparent) Play of Color - The coverage of the play of colors is important, there should be no patches of inferior colorless opal (potch) in a gem class or high grade opal stone.
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Play of Colour |
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Play-of-color of the opal and is the most difficult factor to judge for any opal and does make a difference to the price for the end user.
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Opal Pattern Variety |
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Good patterns of the diffracted colors when combined with brilliance have an enormous impact on the value. Pinfire and small type patterns are less desirable than broad patterns or large flashes. Distinct patterns such as rolling flash, straw pattern, Chinese writing, ribbon and harlequin are very rare and considered collectors' items, almost all opal displays some form of pattern, you will never find two opals that are exactly the same. It’s the amount of color and its intensity that makes it better than another. In particular, many of the top quality pieces of black opal from Lightning Ridge show their strength through a distinct recognizable pattern play of strong colors, some of the various patterns are listed below. Harlequin Pattern:This is the most prized of all patterns and is very rare, the colors in this pattern are more or less arranged like checker board squares. Floral Pattern:Possibly the most common of all nice patterns and it’s name is derived from it’s likeness to a floral dress material, this pattern covers a broad range of design and color. Pinfire Pattern:Closely resembles a mass of pinheads in different colors. Palette Pattern:Like an artist’s palette! Rolling Flash:Usually consists of one color that travels across part or all of the stone as it is moved. |
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Formation of an Opal
What causes the colours in opal?
How does opal form?
Opal is a precious gemstone which is predominantly found in Australia. Scientifically, opal is known as SiO2•nH2O - an amorphous mineraloid comprised of hydrated silicon dioxide. (A mix of silica and water). In terms of overall body tone, opal's appearance can range from colourless, white, and light grey through to dark grey and black. However the truly unique thing about opal is that it displays all the colours of the spectrum in a "play of colour", resulting from the interference and diffraction of light passing through tiny silica spheres in the microstructure of opal. This means the bright rainbow colours of opal, as they appear to the human eye, will move and change dramatically depending on the angle at which the stone is viewed.
The color of an opal is a magnificent thing. Unlike any other gem, opals can display all the colours of the rainbow in an iridescent, moving pattern of red, green, blue, yellow, purple, aqua, pink, and any other colour you can imagine. The pattern and arrangement of the colour which is displayed in an opal can take on many beautiful forms, and the movement of color across the face of a stone is known as the 'play of colour'. This captivating miracle of nature has been admired by people the world over for centuries, and highly sought after for use in jewelry, museums and collectors' pieces. The opal is arguably the most beautiful of all gems - at the very least it is highly unique and a true treasure of the earth that formed during the Cretaceous period, about 65-110 million years ago (Dinosaurs are 20-30 million years old).
But what causes the colors in an opal? What is different about opal that makes the colours dance and play across the face and burn in every colour of the spectrum? The answer, put simply, is the diffraction of light. Much like a prism, which can refract white light and produce a rainbow effect, opals diffract the white light which is coming from above, displaying those amazing opal colours.
Definition of an Opal
A definition of opal:
“An amorphous non-crystalline gem mineral solidified from gelatinous or liquid silica deposited in cracks and cavities left by decaying vegetation, wood, crustaceans and bones. Very valuable in its ‘black’ forms and containing a reasonable content of water. Chemical symbol: SiO2 plus H2O. In higher grades of opal the water content can be as high as 10%. Refractive Index of 1.38 - 1.60 and a hardness of between 5.5 to 6.5 on Mohs’ scale.”
Opal is one of the world's most beautiful gemstones. Opal is Australia’s national gemstone - over 95 per cent of the world’s precious opal comes from Australia.
While several different types of opal are present in nature, there are two main varieties precious opal and common opal or potch and Precious opals that exhibit the characteristic play-of-colour, and is comparatively rare. Opal that is dull and valueless is called common opal. Common opal occurs in abundance throughout the world. Common opal and potch do not exhibit a play of colour. When common opal is found in association with precious opal, it is known as potch. About 95% of all opal mined from the opal fields is common or potch, that is opal that is basically one coloured, i.e. white, grey, black, and is only suitable for backings for doublets or triplets. Of the 5% that has some colour, about 95% is only of mediocre grade - therefore only approximately 0.25 per cent has any real value at all. Of the precious opal found 99% of the precious opal comes from Australia.
Australian opal is referred to, as “sedimentary opal” because it’s host rock is predominantly sedimentary rocks of the Mesozoic Great Artesian Basin. Precious opals usually contain around 5-10% water (for example Lightning Ridge) and consist of small silica spheres arranged in a regular pattern. Inferior opal contains higher percentages of water up to 20% water (for example Cooper Pedy opal), and with the higher content of water comes with less strength. An opal’s hardness of 5.5 to 6.5 on Moh’s scale puts it about half way down the range (roughly a little harder then glass). It has a specific gravity of 1.9 to 2.3 depending on the amount of water present.