Glass-Filled Diamond 'Polki'
by Gagan Choudhary
Glass filling in diamonds to improve their apparent clarity is known for few decades where cleavages and fractures of a diamond are filled with a material of high refractive index (closer to that of the diamond). Commercially, the glass filling of diamonds was first introduced by Zvi Yehuda of Israel in 1980s, while later on the treatment was performed by many other treaters. And since 2003, similar glass-filling started to apply on low-grade rubies too, which became the most controversial gemstone treatment to appear in the past decade. In recent times, in addition to the regular cuts, glass-filling has been applied on 'polki' cut diamonds (figure 1) as well, and that too in large quantities. Most of these glass-filled 'diamond polkis' are being sold undisclosed, especially in 'Kundan-Meena' jewellery (figure 2).
Figure 2: A typical 'kundan-meena' necklace set with ear-rings. Note the metallic reflections from the diamond polkis, which is caused as a result of use of metallic foil.
A brief on 'polki'...
'Polki' is basically a simple, crude and ancient form of a rose cut (figure 3), which today is commonly used in 'kundan-meena' jewellery. Following are three common types of closely-related forms of flat cuts used in such jewellery:
Polki: This is a flat cut diamond with two parallel surfaces being table and a base (figure 1 and 3, centre), while the crown usually is composed of eight facets; this number however may vary depending on the cutter to save the weight.
Parab: This is quite similar to a polki, but the difference lies in the pattern of facets which are slightly concave against the flat ones in polki. In addition, the crown portion is quite small as compared to the polki.
English Polki: This is comparatively more refined and symmetrical cut than 'polki' or 'parab' and usually contains 24 facets (figure 3, right).
Figure 3: Common types of flat cuts used in kundan-meena jewellery include rose cut (left), polki (centre) and 'English polki' (right). All three forms of flat cuts are commonly filled with glass; polki being the more common one.
These flat cuts are however backed by metallic foils to enhance the overall brilliance and fire which is a common practice and a method of setting a diamond or any other coloured stone in 'kundan-meena' jewellery.
The 'polkis' are usually fashioned from the chips derived during cutting polishing well formed crystals or from flat crystals, or even rejections which cannot be fashioned as regular cuts. These usually contain higher numbers of fractures and cleavages, which are filled with glass to make them usable in jewellery.Filling reduces the visibility of fractures and improves overall clarity.
With unaided eye, a filled diamond may appear slightly greasy with a very slight yellowish overtone; in stones with many treated fractures, the yellowish overtone is more apparent. Examination of the fractures and cleavages with a microscope is the best means of positively recognizing the enhancement. These glass-filled polkis can also be detected in a closed back setting in the jewellery. The advantage here is that the polkis are backed by reflective metallic foil, which reflects light back into the diamond once it transmits through, and therefore, typical flash effects can be observed with ease.
Under the magnifying lens...
As any other filled stone, observation under a microscope or a loupe with proper lighting is the key to identify these glass-filled polkis. The features to observe are already well-established; however, these are being reviewed here again:
Flash Effect: This is the most obvious characteristic seen in filled stones. Filled fractures typically display blue colour flash at one angle or illumination which changes to yellow orange on turning the stone or changing the angle of viewing or on changing the illumination (figure 4 and 5). Often, pink, violet and green colour flashes are also observed which are dependent on the exact composition of the filler and illumination. The flash effect is best visible when observed at angles parallel to the fracture, preferably using a fibre-optic light, especially when these polkis are set in jewellery.
Figure 4: Typical flash effect seen in glass-filled diamonds includes violet-blue-pink (left) which changes to yellowish-green (right) on change of illumination or angle of viewing.