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Opal is the gemstone intimately connected with Australia, and rightly so, as more than 95% of the world's opal is mined there. From its discovery in the 1880s until now, this beautiful gemstone has fascinated the world with its play of color and light.
Most people when they think of opal they think of white or milky opal, now often called light opal. This opal has its colors in a light, often white, body tone. White opal is the opal usually found outside Australia. However, other forms of opal - black, boulder and crystal - occur in Australia, and are usually more sought after, as their internal fire and dance of light is better seen. All three have their own beauty, and like all opal, preference is in the eye of the beholder. Unlike other gems, whose value can be quantified by weight, clarity, flawlessness and other objective parameters, opal is a subjective gemstone, whose beauty and value doesn't fit a formula. Like art, what's important in judging an opal is how much you like it, not what someone else deems it to be. However, again like art, the more people that think the same, and the rarer the stone that has this high appeal, the more expensive it will be. (That being said, there are some objective parameters to look for when buying valuable opal; these are elaborated on below.)
In addition to Australia, Opal is also found in Brazil, Guatemala, Honduras, Mexico and even the US, China and Japan, and in Europe Hungary which may have been the origin of the earliest examples of opal jewelry.
What opal is and how it is formed
Opal is different to most other gemstones, and similar to many semi-precious stones in that it is not a crystal structure, but a more amorphous mineral. Due to weathering of silica-bearing rocks, silica, in a semi-fluid gel form, infiltrates small openings in other structures, ranging from fossils to hard-packed earth, and hardens over time. This silica is primarily marine, and the large parts of southern Australia covered by the sea during the Cretaceous (65 - 140 million years ago) enabled a copious deposition. Most opal is in non-gem quality form, called potch, but a small percentage achieves the level of precious opal.
Opal contains tiny spherical crystals of the mineral cristobalite, mostly in a lattice formation, and it is the interplay and diffraction of light with these crystals, through the transparent to semi-opaque silica, that gives opal its color and flash. Larger spheres result in diffraction at the red end of the spectrum, and smaller spheres diffract at the blue/green end. The more even the spheres, the greater the areas of single color.
Opal's chemical composition is SiO2..nH2O, hydrous silicon dioxide. Its hardness is 51/2 - 61/2, on the MOH scale (which is pretty deceptive, as although sapphire is 9 on this scale, and diamond 10, diamond is in fact well over 100 times as hard as sapphire). A strange trait of opal is that it contains water, at around 5-10% or more, and so can vary in weight and composition. High temperatures can actually evaporate some of this water content, and other materials can leach out the water, so care needs to be taken with opal.
Four main types of gem quality opal are recognized - light (white or milky), black, boulder and crystal.
Light opal can vary from clear to grey, but the most common gem grade is white or milky opal, where the background color is a semi-opaque white. Within this, as with all opals, all the colors of the rainbow may appear. Due to the whitish background color, the flash may not appear as strong as with some other opal types, although the underlying diffractive structure is the same. In Australia, Coober Pedy is the main source of light opal, although some is still mined in White Cliffs, New South Wales, where opal was first discovered in Australia.
Black opal is the most valuable opal, due to the contrasting nature of the dark background and the color flashes, and rarity. Black opal comes almost exclusively from Lightning Ridge in New South Wales. As most black opal exhibits green and blue colors, when red is present the opal is very valuable. The color in black opal may only be present in a thin top layer, and so have a generally similar appearance to a doublet or boulder opal; but this does not dimish its value. Black opal is technically a form of dark opal, where the background color is black, rather than brown or gray; all forms are commonly referred to as black opal.
Boulder opal is opal formed in ironstone, which is left on as a backing. The actual opal part of a gem boulder opal may be very thin. Blue is the predominant color in boulder opal, and once again red, when present, increases value. Boulder opal mostly comes from Queensland, between the outback towns of Winton and Quilpie.
Crystal opal is a form of either light or black opal where the background color is transparent, or virtually so. Andamooka in South Australia is home to some of the best crystal opal; opalized fossils are more common here than elsewhere as well.
Other forms are matrix, doublet and triplet. Matrix is a natural form, where the opal is spread through a background rock and so doesn't have large solid flashes of color but small to large dots of color. Matrix may be treated to enhance the background contrast. More artificially manipulated are the two manufactured varieties. A doublet is a thin slice of opal glued to a backing of potch or ironstone, (and occasionally glass), resulting in a thicker gem, and therefore more settable. Doublets can be quite beautiful - they are in fact replicating boulder opal - but are less valuable than an equivalent solid opal. A triplet is a doublet with a third layer, usually clear quartz or glass, attached to the front. Again, these can be quite beautiful, but are less valuable - and naturally, less costly - than doublets, as they can be made with less opal.
Look for the background color to continue through the colorflash area to distinguish black opal from the layered varieties. The background of boulder opal is usually brownish, and uneven.
Australia's opal fields
Opal is found in several places in South Australia, New South Wales and Queensland. The most famous field is Coober Pedy in South Australia. Opal mined here is light opal, primarily in the non-crystal form. Coober Pedy was once the world's major supplier of opal, but its role has diminished (though still important) due to a lack of significant new discoveries. This tiny and truly outback town is easily visited, including as a stopover on the Ghan from Adelaide to Alice Springs, for those wishing to experience first-hand opal mining, and maybe try their hand at noodling, or sifting through old mullock heaps for overlooked color. Much of Coober Pedy is underground, including hotels, homes and other businesses. Quite an experience!
Also in outback South Australia is Andamooka, a significant producer of crystal and matrix opal, and well-known for its opalized fossils. Mintabie, a little north of Coober Pedy, was a major supplier but now is a minor producer.
Lightning Ridge in central New South Wales is now Australia's most significant opal producer, in part due to the quality of opal mined there. Lightning Ridge produces the highest quality black and crystal opal in the world. In addition to opal, the Lightning Ridge area is highly fossiliferous, and the local ammonites (a fossil not a religious sect) are a popular souvenir in both original and jewelry form. Like Coober Pedy, Lightning Ridge is relatively easily visited, with plenty to see. White Cliffs in far western New South Wales is Australia's original mining district, but now is pretty well played out.
Queensland is home to boulder opal, which is found sparingly in a broad swath of central Queensland from a little south of Quilpie to Winton in the north. While most opal is mined in traditional underground fashion, Queensland boulder opal is dug using bulldozers to open cut through the field, and is not so attractive to visitation.