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Boulder Opal or Opal Doublet?

Alan Hodgkinson explores a tricky opal doublet

Opal pendant
Fig 1: Solid opal or opal doublet (composite)?

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.

The intriguing thing about this specimen is that a look at the underside (Figure 3) shows an ironstone rock matrix, (typical of Queensland 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.

Opal pendant, side view
Fig 2: Opal pendant, side view.
Opal pendant, rear view
Fig 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.

Inert rear
Fig 4: Inert seam to the rear.
Fluorescent front
Fig 5: Strong fluorescence to the front.

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.

Alan Hodgkinson - Gemmologist, Tutor and Author


  • Alan Hodgkinson FGA
  • Gemmologist & Author
  • Portencross, UK


  • Gems / Gemmology (primary)
  • Jewellery

Intended Audience

  • Appraisers / Valuers (primary)
  • Gemmology Students
  • Consumers


  • June 2020
  • 1.5 mins reading time
  • Dale-Chall readability level:
    Easily understood by an average College Graduate
  • 2 mins speaking time

About the Author

Alan Hodgkinson - Author and Gemologist

Alan Hodgkinson, FGA

Alan Hodgkinson has lectured in many countries on gemmology and is the recipient of several awards relating to his dedicated study of the gem world. He is the author of the world-renowned book ‘Gem Testing Techniques’, the gemmologist's ‘Bible’ of the 21st century. He is the President of the Scottish Gemmological Association, holds a Honorary Membership with many major gemmological associations worldwide and is a Freeman of the City of London.


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