FROM THE PRESIDENT OF THE OPAL ASSOCIATION - PAUL SEDAWIE
I have been concerned for a while about the amount of treated Welo opal being passed off as natural black opal. There are numerous treated Ethiopian opals wrongly listed on the internet. Equally important they are also been sold in Australian retail shops and unfortunately also on the opal fields. I asked Natasha Patel, an honours graduate in gemmology (AIGS Thailand) to research on how to tell if the stone is natural or treated one is on the opal fields and/without access to laboratory testing equipment.
This is a copy of Natasha Patel’s research report - Natural Black Opal - Are You Asking The Right Question?
The words “Natural Black Opal” can be sometimes not as clear as we believe it to be.
Is the Opal Natural? Yes
Is the Opal Black? Yes
Nowhere in these words does it tell us if the black in the black opal is of natural origin. The market has been flooded with so called black opal in the last few years in the form of dyed / smoked / sugar-acid treated black opal from Ethiopia. The literature on these treatment process is vague and incomplete. The industry is resorting to ad-hoc methods to determine the treatment. The aim of this research, analysis and discussion is to help determined if the black in the black opal is the result of man-made treatments and processes.
The correct question to ask is: IS THE BLACK OPAL TREATED?
Why ask this question? VALUE. Customers are being duped into thinking they are buying natural UNTREATED black opal from Australia when all they are really buying is most often a crystal opal that has been significantly altered to produce a black body colour. The price difference between treated and untreated black can be as much as 500%.
https://www.gia.edu/doc/WN11.pdf (page 79) presents before and after pictures demonstrating the enormous difference in what originally came from the earth and the end result. Beware, even rough opal can be treated to change body colour from transparent crystal/white to black or a shade there of.
Treated black opal from Ethiopia can cost in the wholesale market about $10 per carat. An untreated black opal that look exactly the same may cost up to $5000 per carat and sometimes more.
Black Opal in the trade is sometimes referred to as dark body toned stones. Natural Untreated Black opal predominantly comes Australia but also has been found in Ethiopia, USA, the Czech Republic, Hungary, Mexico and most recently Indonesia.
For the miners, wholesales and traders in the opal industry, the issue of opal that is TREATED to become black being confused with opals that are NATURALLY black is one of upmost importance. Omitting the truth in this case is as good as lying. People who are in the market of buying opals are being taken advantage of through misinformation and lack of knowledge of what questions to really ask. Some may say that TREATED black opal has its place in the market. I, personally and most suppliers of opal agree. However it imperative that its market share is not based on false and misleading information.
The aim of this report is to deal with the following issues:
1. Understand that a NATURAL black opal may TREATED to change the body colour of the stone from transparent to black. The key phrase to keep in mind is UNTREATED natural black opal.
2. Giving people who may be inexperienced in black opal some guidelines and techniques to recognise and distinguish between treated and untreated black opal.
3. In the least be cautioned that body colour of a black opal may be a complete and direct result of treatments.
4. A prominent lab may not be available on or near the field/marketplace when opals are purchased.
5. A black opal stone may not warrant the fee that prominent labs charge to check for treatments. 6. There may be no access to larger gemmological equipment such as a microscope, UV spectrometer etc.
A sample of 15 stones were tested in simple (on field) ways to come up with ideas on how to detect the treatment of natural black opals. You can find the results at the end of this report in Appendix A. The samples included black opals (treated and untreated) from Australia, Ethiopia and Indonesia. Please note that gemmological tests diagnostically prove that all 15 sample are NATURAL.
The tests conducted use simple tools that one may carry with them or have access to when purchasing gems such as:
Black opal is one of the rarest gemstones on earth. Begin with be caution
- When the stone is highly domed
- When the stone is colour through and through (the play of colour is visible on front/back and down the sides of the stone.
The combination of the high dome and full colour is extremely rare in natural UNTREATED black opal.
1. BODY COLOUR AND MATRIX VISIBILITY WITH LIGHT AND LOUPE
a. For natural UNTREATED black opal strong light and loupe will show that the body colour is NOT really black. It’s generally very dark brown or grey. If you are distracted by the colours, concentrate on a spot with no or least amount of colour.
b. All untreated “black opal” samples had some part of the matrix (host rock or colourless/common opal) and inclusions. The difference in hardness of matrix and coloured opal is quite significant and treatment can result in fracturing of the stone.
BECOME CAUTIOUS IF THE BODY COLOUR IS JET BLACK AND THERE IS A COMPLETE LACK OF MATRIX. Please note that presence of matrix does NOT mean that it is NOT treated (some treated black opal may have matrix). Samples 3 and 5 in Figure 1 are good examples of how the body colour can be jet-black.
2. CHANGE IN BODY COLOUR OF STONE WHEN USING TRANSMITTED LIGHT (TORCH LIGHT SOURCE FROM BELOW)
This test is quite simple and can be a starting point to determine if you should become cautious about the source of the black in the black opal. Use a touch with white (yellow in colour and includes the entire visible spectrum) light. The torch and the stone be as close as possible. View the stone from all sides (flip it to have the back of the stone facing upward). A good test for stones that are small in size.
a. The body colour in ALL of the treated Ethiopian samples became completely transparent (see through) when observed with transmitted light - light source (torch) from below.
b. More importantly was the shade of transparent colour visible - a distinct cherry red.
c. Other naturally black/dark opal (when transparent) display a more yellow/grey transparent body colour.
d. All untreated samples were only partly transparent and in most cases completely opaque.
BECOME CAUTIOUS IF THE TRANSPARENCY OF THE OPAL CHANGES CHERRY RED WHEN LOOKING WITH LIGHT SOURCE FROM BELOW.
The differences in colour of transparency is quite obvious from photos below.
3. SPECIFIC GRAVITY TEST
Specific gravity measures of the weight of the stone relative to the size of the stone. All gemstones have differing specific gravities and this test is used habitually in all gem laboratories large and small. The formula is:
The specific gravity of opal is an average of 2.12 with a range of (1.75 to 2.23). The samples of treated Ethiopian black opal consistently yielded numbers closer to the bottom end of the scale (see Appendix A). They all yielded results well below the 2.12 average. ALL Ethiopian treated opal yielded S.G. closer it the 1.75 mark and ALL the untreated black opal yielded S.G. closer to the 2.0 mark.
There are many Do It Yourself resources on the web to construct a specific gravity kit. Respectable gem testing equipment manufactures also kits for sale at a reasonable price. Use a carat scale that is readable to at least 0.01 carats (2 decimal points) and has a resolution of at least 0.01 carats (accurate to .01 of a carat). BE CAUTIOUS WHEN SPECIFIC GRAVITY OF THE STONE IS CLOSER TO 1.75 THEN TO 2.12.
4. DUNK IT – THE “IMMERSION WAY”
Completely immersing the stone in water in a neutral coloured container definitely makes the task of judging treatment much easier. It may not be fool proof but its close.
Dye and smoke marks (black colour concentrations) become more visible. Be sure to look at the stone under water and with strong white light immediately with the aid of a loupe/visor. Sometimes the marks that diagnostically confirm treatment seem to disappear the longer the stone is sub-merged. The body colour becomes darker (black becomes blacker) and therefore the play of colour appears stronger which results the marks not standing out as much.
The immersion and strong light technique also helps to highlight the true body tone of the stone. The following images and explanations will give some indications of what to look for.
5. YOUR PHONE CAMERA (Macro lens optional)
The cameras on phones nowadays are well equipped to take zoomed in pictures. Macro lens adapters for phones are available and affordable. It can give the extra edge needed to examine the stone close-up so that informed decisions can be made about treatment.
The best photos are taken with zoom feature while maintaining a few centimetres distance from phone to stone. Be sure to immersed the stone in water and have strong light close to the stone itself. Remember the longer the stone is immersed in water, the less visible the dye/smoke marks.
If there is a hefty price attached to the stone and confusion about treatment remains, it is recommended that a gemmologist opinion is take into consideration. Remember that not all labs are created equal. Black opal is a rare stone and many labs do not have enough experience with it. It is best to make sure the request for detecting treatments is SPECIFICALLY made and addressed in the certificate.
Gem labs are an important part of the industry but many a times their place just seems a little out of reach. The test methods outlined in this report helps to bridge that gap. For those who are out in the fields, travelling and buying this article gives relatively easy and accessible ways to evaluate the treatment of black opals from all sources. The Ethiopian black opal question is addressed to a greater extent in this report as it is the current concern causing confusion in the market. The gem industry is forever evolving with new finds, new treatments and new demands and customer confidence can only be maintained and increased through cooperation of knowledge and accurate information dissemination. With easy access trading around the world, being at a specific location does not 100% that the opal being sold is of that origin. On several occasions dyed/smoked Ethiopian opal has been offered as Lightning Ridge black on the fields of the mining town in Australia. The treated Ethiopian black opal has reach all localities which produce untreated black opal including Australia, Mexico and Indonesia. Find trusted suppliers and use this study as a guide to assist in recognising the signs of TREATED (dyed/smoked) black opal.
With the exception of the specific gravity test, suppliers can be requested to provide photos of black opal available for sale outlined in this report. Included in your request should be:
Be alerted when:
Closer inspection of the stone reveals that the body colour is JET BLACK and lacks matrix and potch.
The specific gravity of the stone is closer to the minimum range of 1.75.
While the stone is immersed in water (aided with strong light source and loupe/visor) distinct dye/smoke marks become apparent almost immediately.
These test collectively can diagnostically prove that the black on the black opal is NOT of natural origin.
Immerse the stone fully in water to make it easier to see evidence of treatment.
Utilize the zoom function on phone to get clearer and focused pictures of areas of stones that show evidence of treatment.
Construct or purchase a portable specific gravity kit.
Purchase a torch with “white light” (full spectrum and is yellow in colour).
Make sure your research is from reliable, experience resources
Article written by Natassa Patel from Gem Certify
I have being around Australian opal since my teens. After graduating my MBA I joined the family business and for the just over 16 years have been trading and selling opals in cut and rough around the world. Recently I completed my accredited gemmology program for the Asian Institute of Gemological Studies (Thailand) giving me a unique perspective to combine trade experience with the scientific world of gems, specifically opals. Bridging the gap between the gem labs and the traders of gemstones is my main priority with the establishment of my gem certification and consulting services on the Gold Coast. My objective is to create resources and give humble insight into issues that will improve the industry as a whole with key emphasis on increasing consumer knowledge and information. As a member of the following associations I can pool together my learned experiences and those of others: The Opal Association, Gemological Institute of Australia, Jewellers Association of Australia, International Coloured Stone Association and the Jewellers Vigilance Committee.
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