Although pearls come in a variety of colors, high-quality South Sea gold and Tahitian black pearls are rare after all. Therefore, colored pearls that have been processed by dyeing, heating, and irradiation are often seen on the market.
Some color treatments are easy to identify, and the traces of dyeing can be seen with the naked eye; however, with the development of treatment technology, many treatments are difficult to identify with the naked eye. The most confusing of these is the dyeing of South Sea gold pearls.
Comparison of dyed gold pearl(obvious color spots visible to the naked eye) and natural South Sea gold pearl
The laboratory uses microscopic observation, spectroscopy testing and fluorescence observation to distinguish natural and color-treated pearls.
1. Microscopic observation
Some dyed pearls have color accumulation in the punched area and surface defects. This is usually seen in low-quality dyed gold pearls; high-quality dyed gold pearls are almost indistinguishable from natural gold pearls in terms of appearance and surface characteristics.
Color accumulation in the pores and surface defects of dyed gold pearls
2. Ultrashort-wave ultraviolet fluorescence
The fluorescence reaction of natural color South Sea gold pearls under ultra-short wave ultraviolet light is none to medium intensity blue, while dyed gold pearls may show abnormal fluorescence, such as strong green fluorescence. However, some dyed gold pearls do not have special fluorescence reaction, so it is necessary to combine other data for comprehensive judgment.
The reaction of gold pearls under ultra-short-wave ultraviolet fluorescence: the first row is natural gold pearls, and the second row is dyed gold pearls
3. Ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrometer
UV-Vis-NIR spectrometer is one of the most important means to identify pearl color. Natural gold pearls have absorption near 280nm, 365nm and 426nm. The absorption peak position of dyed gold pearls is not only offset, but also the intensity of the absorption peak is significantly different from that of natural gold pearls.
UV-Vis-NIR absorption spectra of natural gold pearls(red) and dyed gold pearls (blue)
4. Raman Spectrometer
The fluorescence reaction of color-treated pearls is different from that of natural pearls, and this is more obvious in Raman spectroscopy (or photoluminescence spectroscopy), and quantitative comparison can be made.
In addition, natural pearls contain natural pigments such as "carotenoids", which have corresponding Raman absorption peaks and can be distinguished from artificial dyes used in color treatment.
Photoluminescence spectra of natural gold pearls (black) and dyed gold pearls(red)
