Plastic Coating Of Gemstones

(via ICA Lab Alert, No.3, 1987) AIGS writes:

Details
In the past 3 years, gemologists at AIGS in Bangkok have encountered an unusual type of assembled stone in which an inferior specimen is coated with colored plastic. All of the stones treated in this manner have been of Burmese origin and so it is believed that the treatment is probably done in Burma.

One type consists of a poor color jadeite cabochon coated with a thin layer of rich green plastic. The coating covers all surfaces of the cabochon except the bottom. After coating, the stone appears to be of very high quality.

Another type is a light color faceted ruby coated with red plastic and then repolished. This gives the appearance of a fine ruby.

The third type seen is a white star sapphire cabochon entirely coated with red plastic. The gem then appears like a beautiful star ruby.

Detection
Although extremely deceptive to the naked eye, these plastic coated stones are readily identified under magnification. They may be dangerous to the trade, though, because their appearance is so natural that unsuspecting dealer might not even check them with the loupe. One unaided clue is provided by the slightly warm and plastic-like feel of the stones. This, however, is very subtle.

Identification of plastic coating is made with the microscope. In the case of the jadeite, as the plastic does not cover the stone entirely, it may be seen to peel away from the stone in places along the girdle. In all types, gas bubbles may be visible in the plastic coating, particularly in the star ruby type, where the coating was thicker. Color swirls could also be seen in the star ruby type. Judicious use of the hot point will, of course, also reveal this fraud.