Cultured fresh water pearls (CFWP) are frequently dyed with various substances for creating greater diversity and fulfilling the modern taste demand of colored pearls. Many colors look obviously unnatural to the eyes of experienced gemologist. Still enormous portion of production is only moderately dyed. Raman/PL spectrometer can be used for detecting organic polyene compounds which give the color of naturally colored freshwater pearls.
All natural colors of cultured freshwater pearls (CFWP) originate from a mixture of organic pigments called polyenes. Polyenes have very strong Raman scattering reaction for 532 nm laser due to an effect called “resonance-enhanced” Raman scattering. The typical spectrum of naturally colored CFWP exhibits a strong and wide general luminescence hump together with peaks originating from aragonite and polyenes. Polyene peaks are not seen in white pearls, but even a subtle off-white color contains enough pigments for positive detection. Any deviation from these reference spectra has to be considered as highly suspicious.
TIP: Pearls may exhibit very strong fluorescence masking all Raman features when initially exposed to green laser. This fluorescence may be reduced by usign built-in Photo-Bleaching feature of GemmoRaman software. Usually a less than one minute photo-bleaching procedure reduces general fluorescence enough for allowing better resolution of Raman peaks.
Dyed CFWPs show various photoluminescence reactions depending on chemicals used for the treatment. Some moderately dyed samples may exhibit both polyene peaks and additional features caused by dyeing agent. The following image shows a plethora of various colored dyed CFWP reactions.
Danilo Bersani, Pier Paolo Lottici: Applications of Raman spectroscopy to gemology, Analytical and Bioanalytical Chemistry (2010)
Karampelas, Fritsch, Sklavounos, Soldatos: Identification of treated-color freshwater cultured pearls
Laura Bergamonti, Danilo Bersani and Pier Paolo Lottici: Raman spectroscopy in corals and pearls