Glass-Filled Polki-Cut CVD Synthetic Diamonds
The term polki refers to a flat-cut diamond that is a simple and ancient form of today’s “rose cut” and has been popular in traditional Indian kundan-meena jewelry. Since most commercial-quality polkis are fashioned from flat rough crystals (macles, for example) or chips derived from the cutting of larger crystals, they often contain cleavages or fissures opening on the surface, making them even more delicate. In the past decade, the trade has been flooded with polkis filled with high-RI glass to improve their clarity as well as durability, and these have been widely used in kundan-meena jewelry.
Recently, the Gem Testing Laboratory (GJEPC) in Jaipur received three light gray to brown polki-cut diamonds (figure 1) with square profiles for identification. They weighed 0.27–0.29 ct and measured 6.86–7.41 mm long and 0.36–0.41 mm thick. The client informed us that the polkis were natural but wanted to know if they were glass-filled. On initial examination under a microscope, glass filling in all three was evidenced by color flashes (figure 2), typically blue, violet, and pink, along with some crackling effects within the fissures and cleavage planes in two directions, intersecting each other at almost 90° (octahedral cleavage in diamond). The presence of glass (containing lead and bromine) was further confirmed by energy-dispersive X-ray fluorescence (EDXRF) analyses. These polkis also displayed a few dark brown grains, some of which were associated with stress cracks. No attempt was made to identify the nature of these grains.
Considering our past experiences with polkis having square profiles, further tests were performed to determine a natural or synthetic origin. When viewed under crossed polarizers, all three polkis displayed a checkerboard strain pattern from the top and sub-parallel columnar patterns from the sides. Infrared spectra confirmed all three specimens as type IIa; DiamondView imaging displayed orange fluorescence, but no distinct growth patterns could be resolved. No phosphorescence was detected in any of the samples. Such growth patterns (under crossed polarizers) and fluorescence have been observed previously in CVD-grown synthetic diamonds by this author, as well as reported in the literature (e.g., P.M. Martineau et al., “Identification of synthetic diamond grown using chemical vapor deposition (CVD),” Spring 2004 G&G, pp. 2–25). Furthermore, photoluminescence spectra using 532 nm laser excitation revealed a distinct silicon vacancy–related peak at ~737 nm, even at room temperature (figure 3). This feature is widely used by gemological laboratories to identify synthetic diamonds (both CVD- and HPHT-grown).
In the past we have seen numerous examples of polki-cut diamonds with glass filling, as well as many CVD synthetic diamonds fashioned as polkis. This was the first time we had encountered glass-filled CVD synthetic diamond polkis, though their market penetration is unknown. Although glass filling is not challenging to identify, encountering it in synthetic diamonds could lead to a misidentification as natural, especially among the trade. Since diamond polkis are usually fashioned from flat rough or chips derived during cutting larger crystals, they often display triangular or irregular profiles, while the polkis described here had a square profile. In view of this, square-shaped polkis offer the best yield for crystals with square profiles, such as natural cubic crystal or synthetic diamond crystals grown by the CVD process, which display a square and tabular habit. Therefore, this feature is quite useful in raising doubts, especially when mixed in parcels of polki-cut natural diamond.
