Boulder Opal or Opal Doublet?

Alan Hodgkinson 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.

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.

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.