Gem News International Gems & Gemology, Winter 2014, Vol. 50, No. 4

An Unusual Doublet Set in Fine Jewelry

Beryl and topaz doublet resembling emerald
Figure 1. This green oval resembling an emerald proved to be a doublet with a beryl top and a topaz bottom. Photo by Olivier Segura.
The Laboratoire Français de Gemmologie (LFG) examined a finely made white gold ring set with a green oval faceted stone that measured approximately 20.2 × 14.7 × 8.6 mm and resembled an emerald (figure 1). The stone showed distinct parallel growth zones, visible to the eye, under the table and the crown. These slightly undulating features (figure 2) were strongly reminiscent of Russian hydrothermal synthetic emerald (see, e.g., K. Schmetzer, “Growth method and growth-related properties of a new type of Russian hydrothermal synthetic emerald,” Spring 1996 G&G, pp. 40–43).

Undulating growth zones in beryl and topaz doublet
Figure 2. Slightly undulating growth zones under the table and crown were reminiscent of Russian hydrothermal synthetic emerald. Photomicrograph by Alexandre Droux; magnified 80×.
The pavilion showed fingerprint-like inclusions consisting of droplets that were easily visible with the loupe (figure 3). This feature might lead one to believe it was a flux synthetic emerald, inconsistent with the observations on the crown. Magnification reveals, however, that the fingerprint-like inclusions are actually made up of very small cavities, with bubbles that are clearly visible in the larger cavities. Therefore, the pavilion is actually a natural gem.

Fingerprint-like patterns in beryl and topaz doublet
Figure 3. Fingerprint-like patterns reminiscent of flux synthetics are in fact very small two-phase liquid-gas inclusions. Photomicrograph by Alexandre Droux; magnified 80×.
At the level of the girdle, close examination revealed one area with a separation line, hidden elsewhere because of the closed setting (figure 4). Through the table, small but prominent black spots occurred on a single plane. These were flat bubbles in cement introduced at the separation plane.

Small black spots in in beryl and topaz doublet
Figure 4. These small black spots are flat bubbles in cement along a separation plane. The line of separation was noticeable through the crown in only one area. Photomicrograph by Alexandre Droux; magnified 30×.
Only the crown appeared a weak red under the Chelsea filter. Under short-wave ultraviolet radiation, the upper part was inert while the lower part showed a light blue, milky luminescence. Under long-wave UV, both were inert. Therefore, the green oval was clearly a doublet (R. Webster, Gems: Their Sources, Descriptions and Identification, 4th ed., rev. by B.W. Anderson, Butterworths, London, pp. 457–468). The refractive index was impossible to measure on either the table or the pavilion because the prongs extended into these two areas.
Raman spectroscopy with a 514 nm laser and 4 cm–1 resolution identified the crown as beryl and the pavilion as topaz. Hence the central stone of the ring is a doublet with a hydrothermal synthetic emerald top and a topaz bottom, which is quite unusual.
This case is consistent with an earlier report (J. Hyršl and U. Henn, “Synthetische smaragd-topas-dublette,” Zeitschrift der Deutschen Gemmologischen Gesellschaft, Vol. 60, No. 3–4, pp. 111–112), which noted that the doublets were of Indian origin. The luminescence of the topaz bottom is slightly different in the present doublet, though still within the known variations of topaz luminescence.
Today far fewer doublets are being submitted to gemological laboratories. This specimen’s unusual setting, closed at the girdle and with large prongs preventing access to the table and pavilion for refractometer measurement, suggests an intent to disguise the composite (and partially synthetic) nature of the central gem. Even fine jewelry can be set with gems that are not natural, and gemologists should remain wary of doublets.

Alexandre Droux and Sophie Leblan are with the Laboratoire Français de Gemmologie (LFG) in Paris. Emmanuel Fritsch is professor of physics at Institut des Matériaux Jean Rouxel (IMN) at the University of Nantes in France. Jaroslav Hyršl is a mineralogist and author in Prague.