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A new type of composite

                                    - Beryl and Glass

Author: Gagan Choudhary

(This article was first appeared in the Gems & Jewellery, Vol. 19, No.1, pp 12-14)

As seen for many years, composites are being made using two or three materials joined together along the plane of girdle to enhance the colour and / or phenomena and /or durability. Being a practicing gemmologist at the leading gemmological laboratory of the country, I frequently encounter interesting gem materials made to imitate well known stones. In this line, I encountered two green specimens weighing 2.64 and 7.58 carats, measuring 9.93 x 7.62 x 5.50mm and 13.66 x 10.76 x 7.45 mm respectively (figure 1) represented as emeralds which turned out to be composites. These two specimens were submitted for identification within a period of one month. These composites warrant a description because of their unusual and interesting patterns.

Figure 1: These two specimens, weighing 2.64 (left) and 7.58 (right) carats represented as emeralds were identified as composites consisting of light coloured beryl and deep green glass.

Visual appearance

At preliminary observations, the specimens appeared to emerald due to their typical colour shade (see again figure 1). Both specimens were green of medium to high saturation with fairly good transparency. However, few scattered black coloured inclusions along with feathers and tubes were visible. These features were quite enough to identify the specimens as natural emeralds. But, gemmology teaches us not to conclude a stone unless it has been analyzed thoroughly and hence, we observed the stones under microscope first. The features we saw were shocking!

Magnification Features

Both specimens displayed similar features which were quite interesting, unusual but distinct enough to identify the nature of the stones. When seen from the table facets, the 2.64 ct specimen displayed fine blade like features (figure 2) oriented perpendicular to the table facet and some liquid fingerprints, while 7.58 ct specimen displayed many long tubes and scattered blackish crystals. All these inclusion features are associated with an emerald of natural origin. However on careful examination, with focus of the microscope deep inside the stone, many scattered gas bubbles (figures 3 and 4) were seen. These gas bubbles appeared to be restricted within some specific areas near the pavilion facets. This gave rise to a doubt regarding the origin of these stones.

When observed carefully, fine circular features were also seen which are typically associated with flattened gas bubbles (figure 3, again) seen along the junction planes of composite gem materials. In order to confirm the exact nature, we observed the stones immersed in bromoform using transmitted diffused light. This revealed the presence of many layers stuck on the pavilion facets. The central area appeared to be light coloured or colourless while the pavilion surface areas were deep green (figure 4 and 5). These deep green areas were divided into several smaller units and varied in saturation which followed the surface / facets (figure 4). In addition, fine junctions between various layers were visible in yellowish colour (figure 6) which could have been due to the glue used. Presence of such junction planes however concluded the materials as composites. The magnification features revealed that these composites have been made using light coloured central light coloured material faceted in oval shape which is followed by glueing smaller deeper green pieces all over the pavilion areas.

The central light coloured area and the pavilion areas displayed a difference in the inclusion patterns as well. The central areas displayed features like crystals, tubes, blades, liquid fingerprints, etc which are associated with a natural stone, while the pavilion areas displayed gas bubbles. The inclusion features indicated the use of a light coloured beryl in the centre surrounded by glasses. However, further tests were conducted in order to identify the nature of materials used conclusively.

UV fluorescence

Ultraviolet light plays an important role in the identification of composite gem materials; in these cases also, it was not an exception and an amazing reaction was observed when the specimens were exposed to long wave and short wave lamps. The specimens displayed a chalky bluish glow was following the junction planes used on the pavilion area (figure 7). This fluorescence effect appeared to have been caused due to the glue used and restricted just below the pavilion surfaces.

The magnification and fluorescence features indicated the specimens to be composites but the exact nature of materials used was still under question. To conclude, further gemmological tests were performed. Refractive index measured on table facet revealed the values of 1.580 to 1.588 with birefringence of 0.008, while pavilion areas gave values of 1.520 with no birefringence. These RI values were consistent with those of beryl and glass respectively. Being a composite, specific gravity was not measured as it is of no importance; no absorption features were seen under desk model spectroscope and was inert under Chelsea filter. Therefore, on the basis of refractive index readings and the inclusion features the materials used were identified as beryl at centre (table) which was surrounded by pieces of glass on the pavilion. Although, FTIR spectrum was also measured which also identified the central material as beryl of natural origin.

Conclusions

Although, the identification of these composites was not difficult but careful observations are required. We do not know in what quantities these are being manufactured, but if mixed in packet lots, identification and separation of these will become a real challenge, especially in the routine dealings. These instances reveal innovative minds of the trade who try to develop some new materials on a regular basis for deceiving and / or making good profits!

All photographs and photomicrographs by Gagan Choudhary