Feb 202015
Figure 1: Solid opal or opal doublet (composite)

Solid opal or opal doublet (composite)

Admin: Alan Hodgkinson is a Special Guest Author for the AIJV.  Here, Alan explores a tricky opal doublet.

It would certainly make life easier for the gemmologist/ appraiser if all composite gems were claw set, or even better if loose (unmounted) – wishful thinking of course. In such situations, the doublet joint is so easy to see.

Life was never meant to be so straightforward and there are situations where the junction plane of a doublet is concealed by a collar setting (figure 1). The pendant shown (Figure 2) provides a good example of the problem.

Figure 2: Opal pendant, side view.

Figure 2: Opal pendant, side view.



The intriguing thing about this specimen is that a look at the underside (Figure 3) shows an ironstone rock matrix, (typical of Qυeensland boulder opal) with a vein of opal which appears to match the play of colour of the top surface. The observer is easily tempted by this to assume a single opal specimen.



Figure 3: Rear of the pendant showing ironstone with precious opal streak.

Figure 3: Rear of the pendant showing ironstone with precious opal streak.

A brief exposure to ultraviolet long wave light records an inert response from the opal seam on the underside (Figure 4). In contrast, the topside of the opal fluoresces strongly (Figure 5), followed by a noticeable phosphorescence.

These contrasted luminescent features of the front and back of the piece is so evident, that the query posed is instantly answered: the pendant holds an opal doublet, but the clever choice of the two components is so well matched between front and back, as to induce the response that all is well with the valuable looking gem.
The difference in value between these two options makes it imperative to find the right answer, quite apart from protecting one’s reputation.



Figure 4:  Inert seam to the rear.

Figure 4: Inert seam to the rear.

Figure 5: Strong fluorescence to the front.

Figure 5: Strong fluorescence to the front.










Disclaimer – The AIJV blog is authored by a selection of AIJV members and guests specifically to be able present many different viewpoints on a large variety of subjects. The opinions expressed by the authors are not necessarily those of the AIJV.

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Alan Hodgkinson

Alan Hodgkinson is a well known teaching gemmologist and is President of the Scottish Gemmological Association and past Antonio C. Bonanno Award winner from the Accredited Gemologists Association. Alan's long awaited book 'Gem Testing Techniques' is due out in 2015. The AIJV are proud to have Alan is a Guest Author and we thank him for his generosity.

  4 Responses to “Boulder opal or opal doublet?”

  1. Thanks Alan
    Another of your potentially simple solutions to a tricky problem.

  2. Thank you Alan, 100% agreement with Adrian
    I can’t wait for the book to be released.

  3. Finally got a chance to read this over. Great post. Thanks Alan and Adrian.

  4. Perfect timing, as I have been asked to look at a spoon with an “Opal” set in the terminal. The spoon appears to have some age, but not antique, but the setting looks like its been tampered with.

    Many thanks and I’ll keep you posted.

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