Identification Characteristic of Freshwater Cultured Nucleated “Edison” Pearl

"Edison" pearls as a type of large, rounded, shiny and colourful freshwater cultured nucleated pearls, are getting more and more attention.Their unique purple and bronze colours with metallic luster are deeply loved by people.However, the cause of colour in "Edison" pearls has been questioned, and the dyed imitations gradually appear in the market.In order to identify the "Edison" pearl and its imitations accurately, the authors collected 15 pieces of "Edison" pearl samples with different colours and applied systematic tests on the gemmological characteristics including magnification observation, the thickness of the nacre layer, fluorescent images, UV-Vis absorption spectroscopy and Raman spectroscopy.The cause of colour in the "Edison" pearls, dyed imitations, and purple freshwater cultured non-nucleated pearls were analyzed.In Raman spectra of "Edison" pearl, peaks at 1 121-1 132 cm^-1 and 1 503-1 524 cm^-1 related to the natural pigments of polyenic (polyacetylenic) compounds were regarded as the major evidence on the cause of colour.Dyed imitations could be distinguished by magnification observation, abnormal fluorescent images and UV-Vis absorption spectroscopy.