SOLID OPALS are the most valuable, cut from only the natural stone without any treatment or enhancement. Within this category, there are three kinds of solid opal − Light and crystal opal, Boulder opal and Black opal.
Light and crystal opal ranges from a near opaque milky white to a transparent, translucent colour. The transparent varieties are commonly referred to as crystal and are generally the most beautiful form of light opal due to the amazing play of colour. They do not possess a true crystal structure − the reference to crystal refers only to the translucence nature. This type of opal is mined at Coober Pedy and Andamooka.
Boulder opal can be dark or light and is found in thin veins of sedimentary ironstone boulders. The colour layers are naturally bonded to the dense brown ironstone base to such a degree that it is inseparable. This opal forms in horizontal and vertical cracks in the rock, with better quality boulder found in horizontal seams. The quality of the boulder lessens as the thickness of the opal increases. This type of opal is mined in Queensland.
Black opal is an opaque body ranging from a dark grey to black from which overlying spectral colours are enhanced. This occurs because the dark background of the opal absorbs the otherwise scattered white light developed in light varieties of opal which dilute and diffuse the colour play, thus enhancing brilliance. In most valuable black opal, you can see a thin uppermost layer containing the colour play over a dark devoid of colour base (potch base). Black opal is generally the most valuable form of opal, provided all other things are equal. This opal is mined at Lightning Ridge
Doublet Opals and Triplet Opals:
Doublet opals are a thin slice of translucent opal joined by an epoxy resin to a backing, usually either made from black potch or ironstone. The opal is cut thin, which makes the translucent opal look darker once placed on the dark backing, giving it the resemblance of a black or boulder opal. The purpose of doublet opals is to provide an economical alternative to the gem quality black or boulder opals, as their value is only a fraction of the cost of the real solid opal. They can be distinguished by looking at them from the cross section, where a straight distinct line can depict the different layers.
Triplet opals Triplet opals are made by adding a clear protective quartz or crystal glass top to doublet opals; the slices are usually thinner than a doublet to make them less expensive. The domed cap protects the opal and magnifies the colour and pattern, improving the stone.
Synthetic Opal:
Often referred to as Gilson after one of the manufacturers, synthetic opal is manufactured in a laboratory. The colour patterns are usually amazingly bright and consistent, therefore making it hard to believe that it is real. The opal comes in light and dark shades and the colour pattern is always the same, which makes it different to real opal where the colour pattern tends to be more random. Solid synthetic opal can be easily detected when looking at a cross section of the opal, because the colour and pattern follow the stone in a vertical or horizontal fashion throughout the entire stone.
Treated Opal:
Andamooka matrix can be treated in a bath of sulphuric acid and sugar, boiled over a slow heat over 24 hours to enhance the matrix (darken the material).
Quality and Valuation:
There are a number of factors to take into account when considering the quality and valuation of opals:
Intensity / Brilliance
The play of colour is an important factor to take into account in a stone. A better stone will be clear and crisp in colour, and the shades of colour will be distinct, definite and intense without being cloudy. A darker based opal will generally be more valuable than the equivalent lighter based opal (all things being equal), with an intensely black background opal being the most desirable and expensive.
Colour Range
Choosing an opal can be a subjective choice dependant on colour preference, however when it comes to valuing a stone to sell, certain colours are considered more valuable due to scarcity factors.
- Blue is a desirable and attractive colour but are less valuable because of their relative abundance. Blue can vary from a cloudy light blue to an intense electric blue.
- Green tends to be more expensive and more beautiful, especially when made into doublet opals on a dark background. The supply is less abundant than blue.
- Yellow, although less common in black opal, is not as highly sort after in a light opal. It’s usually paired up with orange or green.
- Orange is a highly sought after colour, second only to red in value.
- Red tones from magenta and scarlet to crimson are highly sought after, especially on a black base. When this is combined with green, orange and blue, it becomes an extremely expensive and high-quality gem.
Colour Pattern
Opals can occur in many unusual patterns, but there are some particular categories of patterns that can be grouped:
- Harlequin pattern is extremely rare and is made up of a patchwork of irregular sized squares.
- Infire pattern is closely spaced pinpoints or specks of colour.
- Flash pattern shows broad irregular flashes of colour which change or disappear as the stone is moved.
Shape
The most desired shape for opal is a domed cabochon oval with a length about 40% greater than the width and the height 60% of the width, which is quite rare to find. These types of opals are priced at a premium, as so much opal is lost in the cutting. More commonly, opal is cut in irregular or elongated ovals, tear drops, or round to maximise the opal’s natural shape. It can also be sliced thinly, as is the case with triplet and doublet opals. Flat or excessively thick stones can devalue a stone, all things being equal.
Stone Size
Opals under 10 carats are the most popular size. Larger sizes do not attain a higher price per carat like diamonds. In fact, the reverse is usually the case. Opal does have a lower specific gravity than sapphires, rubies or emeralds, meaning opal will be a bigger stone size per carat than other gems
Inclusions
Opals with sand or potch on the back of the stone do not decrease the value unless they are visible on the face. Some darker stones can have what we call a window in the stone, where there is a transparent portion of the stone. This can decrease the appeal and value.
To learn more about any of the above types of opals, get in touch with the expert team at Johnston Opals today.
What is Opal?
Australian opal which makes up 95% of the worlds opal, is an amorphous, non-crystalline silica, (silicon dioxide, similar to quartz and sand) a hydrated form of silica (SiO2H2O) in a rigid gel form containing between 1% and 21% water. Precious opal usually contains 6% to 10% water. The water is located in tiny voids between the spheres that are so tiny that you need an electron microscope to see them and they are held so tightly that it is nearly impossible for them to escape.
Silicate minerals in the stone add to its weight giving it a specific gravity of between 1.98 and 2.2 of pure water (precious opal is between 2.1-2.2).
Opals scratch hardness the Mohs scale ranges from 6.0 to 6.5 which is between the hardness of a moonstone and quartz and its refractive index varies from 1.44-1.46
Opal is a closely packed array of billions of spherical particles stacked in a three dimensional grating that has the unique ability to diffract white light into beautiful colours of the rainbow without any impurities. The colour is created when light is split by the voids between the spheres.
The diameters of the spheres therefore determine the maximum size of the wavelength or colour that can be developed. Visible colour spheres must be no smaller than 1500 angstroms for violet, indigo and blue and no larger than 3500 angstroms for orange or red.
Therefore a stone that can display red can display all shorter visible wave lengths, orange yellow green and blue.
Opal in Australia was formed about 60 million years ago when the deserts of central Australia were a great inland sea with silica rich deposits situated around its shoreline. Over time climatic change caused the sea to recede and disappear becoming the great artesian basin. During this time a solution subsequently was deposited in open cavities and cracks in sedimentary rock, not volcanic, around the Cretaceous period or the Age of the Dinosaurs, (explaining the occasional discoveries of prehistoric opalised skeletons, wood and shells).
Where is opal in Australia found?
Opal is found in three states in Australia:
New South Wales, Queensland, and South Australia:
The New South Wales opal fields are situated in flat, arid inland areas, some of the mines are White Cliffs is situated 295km north east of Broken Hill and is the oldest commercial opal field where mining started in 1889 when George Hooley and Alf Richardson discovered opal while kangaroo shooting. The town did not grow until the 1890 drought broke in 1893 when the population grew to 800 and production was in full swing growing to a population of 5000. Mining at White Cliffs continued to be a top producer for 20 years and much of the opal that came out was opalised wood, shells plants, and animals including the famous pseudomorphs (pineapple).?
Lightning Ridge situated 770km North West of Sydney first discovered opal in the late 1880s but its commercial value was not recognized until the great find of 1907. Lightning ridge is currently the major contributor of black opal in Australia. Many of its fields have effectively been mined out such as Three Mile, Thorleys Six Mile, Nobbys Deep Four Mile, Six Mile, Nebea Hill, Shallow Belars. Nevertheless there are many closely located fields.
Coocoran lake opal fields is about 20km west of lightning ridge and was first mined in the depression years, it was the site of a major rush in the 1990s and produced a large amount of highly valuable black nobbies.
Sheepyard field near Glengarry found in 1985, produces seam opal both black and white
Grawin is 42km south west of Lightning Ridge and most the opal is seam opal, light predominantly green in colour.
Glengarry is 4 km north east of Grawin, again a seam opal field
Carters Rush is 5 km north east of Grawin first developed in 1974 and again seam opal
Mehi is an old field discovered in 1924 and is still mined up until today
Queensland is famous for the beautiful boulder opal
Opal was first discovered in 1869 on Listowel Downs in Western Queensland prior to that the world had only known of the white opal from Europe. It took many years for it to be successfully marketed
The Yowah opal fields were discovered in the 1880s being named after the Yowah Creek. Opal from this area is highly renowned because of the famous Yowah opal nut.
Koroit is another opal field 85 km from Cunnamulla discovered by Lawrence Rostron in 1897. Koroit is still one of the best producing opal fields in Queensland famous for its matrix
Black Gate is a small opal field 90km from Eulo discovered in 1894 when it was known as Dynevor Downs field
Duck Creek and Sheep Station Creek were small opal fields discovered between 1889 and 1891. Duck creek was renowned at that time for its fine crystal opal but mining was short lived because of the lack of water
Quilpie is a town which is a central point for the Queensland opal industry, there are many mines situated around it
Eromanga is a town 104km west of Quilpie, the most inland town in Australia. Opal was discovered in the area in the 1870s and this was the main selling point for opal at the time it was the only opal town prior to White Cliffs
Opalton is a field 112km south of Winton discovered in 1886 by George Cragg renowned for the large volume and quality of opal produced.
Mayneside is another area where there are many mines; it is located 45km south of Opalton. It produced quality pipe opal some of which was black.
Kynuna discovered in 1894 by Peter Karaff was a small mine which mining stopped many years ago
South Australia was the first reported state where John Menge, found opal in the Adelaide hills in 1840. However it was not until 1914 when a young boy discovered Coober Pedy whilst searching for water that the opal mining industry began in South Australia
Coober Pedy is the largest opal mine in Australia , situated 1000km north of Adelaide , the name comes from the meaning White man in a hole it was not until the discovery of the Eight Mile mine in 1946 that Coober Pedy made its mark as the opal capital of the world. People who lived in the area adapted to the hot dry conditions by making their homes underground in what is called a dugout. Opal from Coober Pedy is of a light nature.
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Andamooka was discovered in 1930 by Sam Brooks and Roy Shephard it is situated near Woomera. The opal from this area is of a light nature but many consider this opal to be the best light crystal opal available, although very little is mined in this area these days.
Mintabie is another mine situated 30km west of Marla on aboriginal land it is not known when mining first began however, mining was noted in the 1920s, the mine was small in comparison to Coober Pedy but it produced very high quality white as well as black opal in the 1970s and 1980s.
Lambina is 58km east of Marla mining was rumored to begin in the 1920s and early 2000 big finds were again made the opal found was light opal with generally more blue/green.