Metallic See-Through Backing on Omphacite Jade
The Carlsbad laboratory recently received for identification an omphacite jade carving in the shape of a pea pod (figure 1) with an unusual effect created by its backing. Adhered on the back of the carving, which measured 23.71 × 9.67 × 2.59 mm and weighed 4.52 ct, was a thin layer of plastic-like material with a metallic luster. Standard gemological testing revealed a spot refractive index of 1.66 and weak blue fluorescence due to the plastic layer; microscopic observation showed the film in reflected light. A metallic film of 0.10 mm thickness was found to be adhered on the back of the piece to create a metallic look. It had a see-through quality, shown in figure 2. Raman analysis of the thin film closely matched with a plastic used in Mylar.
Analysis for polymer impregnation with Fourier-transform infrared spectroscopy revealed the importance of direction in testing the piece. If the laser was positioned perpendicular to the metallic film so that the laser went through the film, the glue layer would be detected, creating a false positive for polymer impregnation. Analysis parallel to the metallic film proved the piece was not polymer impregnated. Understanding and identifying all components of such a piece is very important in the determination of jade treatment types.
