There has been a lot of discussion about Ethiopian Opals and the treatments involved with these stones that we asked Anthony Smallwood from Australian Opal Association to write an article about his observations.
Over recent weeks a substantial amount of research has been conducted to look at some samples of Ethiopian opal that can be regarded as slightly different from that produced in Australia. Also research has commenced on the “treated’ varieties of Ethiopian Hydrophane precious opal that is appearing online and predominantly EBay.
In late 2013 I received a parcel of four rough or semi-rubbed Ethiopian opals with a request to look at the trace element content of the “Black” portions of the opal.
The samples are best described as being of 10 to 15 millimetres in size. Two of the samples were predominantly light to almost crystal opal with a small 5mm round centre section. Cabochon rub was “black” opal on a white to grey common opal.
These specimens were examined by Scanning Electron Microscope (SEM) techniques in order to determine the content of the ‘black” portions to see specifically if any ‘Carbon’ was present, or if any other elements could be detected. Analysis in this way gives an overall picture of what is present in these materials. It is limited to the bulk content of the material and not all elements are detected.
The preliminary results on these specimens suggest the material was just line normal opal and showed only Silicon and Oxygen in the results. A small amount of Carbon was detected in two of the attempts, however at present the amount suggests more that it was contamination rather than a mineral component of the opal analysed.
Shortly we will re-analyse these samples using more sensitive testing in order to obtain results in the Parts per million (ppm) or parts per billion range. This will give a much better spectrum of the trace elements in this material.
We have also analysed six specimens of “Black” treated Ethiopian Hydrophane precious opal by both SEM and normal gemmological instruments to determine appropriate methods for separating this material from other Australian precious black opal. The testing showed significant and measurable differences in each instance.
Precious Hydrophane opal by definition is a type of opal that will absorb a quality of water when it is immersed. The amount of water absorbed varies from specimen to specimen. Usually when first purchased a lot of this material has a white body colour, and it goes transparent when immersed in water. As such it is relatively easy to change the body colour. By simply adding food colouring to the water the body colour can be changed to blue, blue green, red and other colours.
Needless to say the obvious treatment for this material is to treat it or “dye” the stone black to give the appearance of black opal. This material is being miss-represented both on the “Web”, EBay, as well as some overseas trade fairs. There are reports of processes being used in India to treat this material to a Black body colour, however the actual processes are still to be determined, and we will report them as they become known.
The samples so far processed suggest that apart for food colours, smoke and smoke treatment, the compounds revealed a quantity of Potassium (K) Calcium (Ca) and Aluminium (Al). Again a different profile to what is expected from an analysis of ordinary precious opal from Australia.
From a gemmological point of view the refractive index (RI) of these specimens was on the low side ranging from 1.35 to 1.40. More conclusively the Specific gravity (SG) or density was also measurable and was on average 1.85. Both of these gemmological constants are substantially lower than any Australian Black precious opal. As an example Lightning Ridge Black opal has a usually narrow Refractive Index of 1.45 and a Specific Gravity of 2.15.
Please be aware that these tests need to be conducted by a qualified gemmologist, and care must be taken with the Hydrophane as attempting to obtain a refractive Index reading can lead to the absorption of the liquid used for the test and result in an ugly brown stain on the opal which may not be able to be removed.
The third item to be reported is a trip and presentation to the gem show at Tucson Arizona this year and specifically to report to the International Coloured Stone Association’s Laboratory committee the GILC. Three of us Anthony Smallwood and Terry Coldham representing the Gemmological Association of Australia (GAA), and Maxine O’Brian representing the National Opal Miners Association (NOMA).
The presentation delivered by Terry Coldham involved a discussion on the unique qualities of all types of Australian Precious Opal from Lightning Ridge Black opal, through Queensland boulder opal to the distinctive features of South Australian light and crystal precious opal and including unique Australian fossil opal.
The aim was to instruct our colleagues in various gemmological laboratories around the world that it was indeed possible to separate the origin of Australian Precious opal varieties from otherworld wide opal occurrences. It is hopeful that this will allow identification of the origin of Australian Precious opal to be included on gemstone reports from the important Gemmological Laboratories.
An invitation was also given to members form the Labs to attend an Australian adventure and come to Australian for an educational tour of important opal mining districts in Australia. This is presently being organised with a suggested date of September 2014.
Further as a result of these discussions and presentations several written articles are being prepared which will more formally elucidate the information that is being developed and will be published in upcoming issues of the “Australian Gemmologist” and the ICA magazine “In Colour” so stay tuned.
Anthony Smallwood FGAA, GG, MSc. (UTS).
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