Admin: Alan Hodgkinson is a Special Guest Author for the AIJV. Here is an excerpt from his soon to be released book ‘Gem Testing Techniques’
Nothing could be easier than to identify a gemstone as a composite (doublet or triplet) when confronted by a loose gemstone. A view of the junction plane will normally reveal the dual component quite easily, regardless of the skill of the maker of such counterfeit gemstones.
Typically the junction coincides with the girdle facet of the stone, though in the case of the garnet-topped doublets, the junction is more likely to appear on the crown or pavilion facets. If however the typical doublet is set in a collar setting, the junction plane will normally be unseen, and then comes the task of determining if the stone is a ruby, emerald, diamond, or a doublet, and an appreciable difference in the value rests on the verdict. The ruby and diamond ring shown Figure-1, presents such a problem. The doublet comprises a thin green sapphire crown and a synthetic ruby pavilion.
What is ingenious about this particular specimen is that the first thing one is aware of is the large natural inclusion just under the table facet, and evident to the unaided eye – “surely a natural stone”.
With a loupe deployed, the magnified view Figure 2, reveals what appear as ‘silk’ inclusions, a typical feature of natural ruby. In fact the ‘silk’ lines have been scratched with a diamond point onto the interface of either the sapphire or the synthetic ruby, and they can only be described as ingenious in the way such gemmological evidence has been incorporated into the gemstone.
Joining the two components involves an adhesive of some kind such as an epoxy resin, and pressing the two parts together must be done carefully to squeeze out any air bubbles. Not an easy job, and some gaps in the adhesive can be seen Figure-3, which, with their soft contoured outlines of low relief, distinguish them from bubbles.
As the described inclusions feature all in the one plane, they come into focus together, which should catch the eye of an alert gemmologist.
It is while running the loupe over the stone that the vital evidence comes into view Figure 4. As light passes down through the table and crown facets, they project the illuminated facet shapes down below the surface of the gem. Such projected illumination shapes will register on the interface of the two parts, to give the appearance of repeated crown facets.
The diagnostic effect gives the facets what seems like a picture-framed effect, and this is conclusive evidence of the doublet, even though it is concealed by this particular type of setting. The author first showed these diagnostic features in September 1980.
Portencross, Scotland. November 2010
———————————————————————————–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 opinons expressed by the authors are not necessarily those of the AIJV.