The 95% of the pearls present in the gem market are freshwater cultured pearls in Hyriopsis ssp. Heating is frequently applied to off-colored pearls for their color enhancement. The understanding of the thermal behavior of pearls, would be useful to separate the natural colored from the treat-colored (after heating) pearls. This study presents analysis of the mineral structure and the organic matrix as well as the thermal behavior of Hyriopsis ssp. cultured pearls.
The studied samples were white freshwater cultured pearls in Hyriopsis ssp. which were analyzed with the X-ray powder diffraction (XRPD) and the Fourier transform infrared spectroscopy (FTIR) methods. In the XRPD patterns of all samples only calcium carbonate was identified with the structure of aragonite. The FTIR transmittance spectra of the powdered samples, using the KBr technique, show the characteristic absorption peaks of aragonite. However, some additional shoulders, at about 1662, 1270 and 1172 cm-1, were observed probably due to the organic matter of the pearls. Furthermore, the broad bands in the region between 3600 and 3200 cm-1 are probably attributed to the water content of the pearls. After heating at “low” temperatures (up to 250 ºC), changes were observed only in the FTIR peaks related to the organic matter and the water. No changes on XRPD patterns were observed.
This preliminary study indicates that heat-treatment of the pearls up to 250 ºC, changes only the bands of organic matter and water in the FTIR spectra. With a followed heat treatment, especially of the whole pearls, the critical temperature of their color changes can be found and linked with the decomposition of the organic matter and/or pearls dehydration. Additional studies with non-destructive methods, by which the above changes could be observed (e.g. Raman spectroscopy), will be performed in order a method applicable in gemological laboratories to be found.