Although opal is often regarded as the ‘Queen of Gems’ there is also – as in any royal family – a recognized hierarchy among them. As examined in my Opals, Real and Faux post, the top honors for both rarity and beauty are accorded to the black opals, especially those found in the Lightning Ridge area of Australia. There are, however, other regions where black opal is mined; and therein lie traps for the unwary buyer.
The three major non-Australian opal-mining countries are Ethiopia, Indonesia, and Honduras. However, there are differences between stones from those areas versus the Australian gems. The Ethiopian stones from Shewa have a dark brown body color often referred to as “chocolate” and are prone to cracking. The Indonesian stones are likewise more brown than black and although the South American ones do have a true black body, they rarely exhibit the large dramatic patches of color found in their Australian counterparts.
A new Ethiopian dark-ish opal appeared on the market about ten years ago (ca. 2009) from mines in the Wollo province (sometimes spelled “Welo” or “Wello.”) Wollo/Welo opal is a form of crystal opal, also called hydrophane. As its name implies, these stones react to the presence of water by temporarily losing their color, which reappears as the stone dries out. Wollo/Welo opal does exhibit good color and so it was only a matter of time before some sellers decided to “improve on nature” to make them appear more like true black opals. Most buyers are not aware that it is possible for opals to be artificially treated in order to change their appearance. The most common methods are dyeing, smoking and carbonization (a/k/a sugar treatment.)
Dyed Wollo/Welo opals are exactly as the term suggests: stones that have been soaked in a dye solution. The hydrophanic nature of these stones lends them particularly well to this process. Some dyed stones are ridiculously obvious: jade-like greens, bright oranges or reds, unrealistic blues, all giving the stones a fairly uniform appearance. Others are only slightly dyed, so as to enhance only the colors appearing fairly close to the surface, and those are a bit harder to detect. Laboratory examination under a microscope will reveal characteristics of the dye process, as will cutting into it physically.
Smoke Treatment of Opals
Smoke treatment of opals is not a new process; it has been used for many decades on opals from Mexico which are almost always below gem quality. It’s ridiculously easy to do: Simply wrap the stone tightly in kraft paper, put it into a heatproof container, apply heat until the paper gets charred, let the stone cool, wash it off, and voila! An opal with darker, richer colors. Of course this is just a shallow surface treatment that will eventually wash or wear off, but it lasts long enough for selling purposes although not for professional examination and cleaning.
When many “black” Ethiopian opals began appearing on the market it raised red flags among gemologists, particularly at the GIA, who were puzzled that the suspiciously-dark coloration was not merely on the surface. Was it a new technique? Not really, but the stone itself was “new” in terms of how it reacted to smoking. Just as with a dye process, “charring” is able to penetrate deeply and evenly into Wollo/Welo opals; the only physical way to know for sure if the dark body color is a result of such treatment is to literally cut the stone open. The color is also more stable than dye, because it is not the result of any liquid.
There is a sure way to detect smoking but it involves the use of spectroscopy. Without getting into the weeds too much, a lab can look for a specific “carbon” reading as a result of using Ramen spectroscopy. This particular reading (1150 cm-1) is specific to carbon smoke, a/k/a the residue produced by burning. It will always be present in a smoke-treated opal but does not exist in natural (“non-smoked”) stones. Unfortunately, all too many opal sellers advertise the smoked stones as “natural black Ethiopian opal” or simply “natural black opal” and will furnish bogus ‘certificates’ to that effect. The only real way to protect oneself against buying a misrepresented treated opal is to condition the purchase on getting an independent (not furnished by the seller) lab’s spectroscopic test for the specific carbon reading. That test is not part of the normal examination procedure for opals, by the way, but in the absence of such confirmation it should be assumed that any “black” Ethiopian opal has been treated and thus is not 100% natural as mined.
One legally effective dodge that some sellers use is to advertise treated stones as “natural color black opal” instead of “natural black opal.” Semantically this is correct because, after all, the black color is from carbon which is a natural element (unlike a dye, for example.) Ethically, well, not so much! It is akin to the myriad food products that include “natural vanilla flavor” in the ingredient list; it is not the same as “natural vanilla extract.” Natural vanilla flavor is a flavoring that is made from the fermentation of corn bran, rice bran, or yeast and has never seen the inside of a vanilla bean. On the other hand, “natural vanilla extract” comes only from vanilla beans. So, if you see something advertised as a “natural color black opal” it almost certainly means a smoked stone.
Sugar Treatment of Opals
A close relative of ‘smoking’ is to treat a stone using a combination of sugar and heat. This is not restricted only to Ethiopian opals; it is also often used on the matrix opal that is mined in Andamooka (Australia) and Honduras. Like its hydrophanic Wollo/Welo “cousin,” matrix opal is more porous by nature. Its natural body color is white (“milky”) and sometimes almost clear, which provides far less contrast to highlight whatever color the stone naturally has. In fact, most matrix opal on the market has probably been sugar treated. But unlike Welo opal, matrix opal does not react to water in exactly the same way.
Sugar treatment is fairly straightforward but also a bit tricky. The opal is covered in a sugar (glucose) solution so that it soaks into the uppermost layers of the stone. The stone(s) and solution are then heated (at a minimum of 200 degrees) until the soaking solution evaporates. At this point the sugar will have carbonized around the stone’s surface; anyone who has ever accidentally burned the bottom of a pan while trying to caramelize sugar will know exactly what this looks like! The “caramelized” opal is allowed to cool slowly in its container until the final (tricky) step: The careful addition of a minimum-50% sulfuric acid solution. This sets up a rather dramatic chemical reaction that turns the ‘burnt’ sugar residue within the opal’s body much darker. In the end this treatment is not really very different from smoking because both involve heat and the ultimate resulting material in both cases is carbon. And in both cases the treatment can be detected by use of the proper (and advanced) laboratory methods.
Responsible sellers will correctly advertise their matrix opal stones as “treated matrix opal” as the three Andamooka stones above were. But when the same process is used on the Ethiopian Wollo/Welo stones it is often not disclosed, and even outright misrepresented as actual black opal.
Other Opal Treatments
Another opal treatment that has the side effect of enhancing colors is polymerization. This is commonly used to stabilize an otherwise-iffy form of opal (usually matrix) by filling the minute cracks and crevices with a resin such as Opticon.Here is a piece of matrix opal before and after being stabilized with Opticon. The body color has noticeably darkened and the red has been enhanced.
Out of morbid curiosity I did a quick eBay search for “black welo opal.” It generated 12,013 results, almost all of which are from one or two sellers in Thailand and India. A similar situation exists on Etsy. A common denominator is that almost all are titled as being “Natural black Ethiopian fire opal” or a close derivative thereof. One would think/hope that people who buy stones on such venues or on tv-shopping channels are well aware that they are not getting the ‘real thing.’
The fact that the stone is from Ethiopia should be the first clue, because the only truly dark/black opals come from a very small region near the town of Gashena (the Stayish mine, which discovered a deposit in 2013.) Because the stones are of volcanic origin and thus display a high carbon reading, it is challenging to determine via spectroscopy whether the stone is really from Stayish or is a smoked example from elsewhere. Because of the tendency of Stayish opal to crack (more like “crumble”, actually) during the cutting process, much of it is sold in the rough.
The “chocolate” opals from the Shewa mine are brown, not black – selling them as ‘chocolate opal’ is fine, but treating them and then passing them off as ‘black opal’ is not.
Black Opals from Nevada
I would be remiss if I didn’t mention the black opal that is mined in the Virgin Valley area of Nevada. The three major mines are Rainbow Ridge, Royal Peacock, and Swordfish. These actually are black opal (which is also the official state gemstone) but there is a difference in its formation vis-à-vis the Australian ones: The Nevada stones form in seams of bentonite. The Virgin Valley stones are top quality, and in fact the largest black opal rough in the Smithsonian comes from there. Named the Roebling, it clocks in at just shy of 2600 carats!
Although they are beautiful, the Nevada opals do have a significant downside: Almost all of them are unstable, which means they are extremely prone to cracking. Some people actually store them in water as the sure means of avoiding this. This is why most Virgin Valley stones are sold as display specimens and are not suitable for jewelry. When they dry out, probably about 90% of them will crack. So, they really do fulfil the criteria of being “heartbreakingly beautiful” opals!
(Learn more about the different types of opals in my Jewels That Play with Light: Opals, Real and Faux post.)