Identification of Two Kinds of Seawater Cultured Gray Pearls by Fourier Transform Infrared Spectroscopy and Ultraviolet-visible Absorption Spectroscopy

Abstract

BACKGROUND Akoya pearl is a biomineralized organic gemstone. The mineral composition and microtextural analysis, and the identification method of color formation of this type of pearl are still lacking. OBJECTIVES To identify natural gray pearls from treated-color gray pearls. METHODS The spectroscopic and textural characteristics of two kinds of seawater cultured gray pearls with white or gray nucleus were comparatively investigated by optical microscope, Fourier transform infrared spectroscopy, ultraviolet-visible absorption spectroscopy, and energy dispersive X-ray fluorescence spectrometry. RESULTS The gray pearls with white or gray nucleus had diameters from 5mm to 9mm, the thickness of 300-500μm. The nacreous layer and its corresponding nucleus of the two kinds of gray pearls were composed mainly of biological aragonite, and their corresponding nacreous layers were rich in Sr and poor in Mn. The pearl with the gray nucleus was rich in Mn. The valence state of Mn was increased due to irradiation, resulting in gray of the pearl core and the whole pearl. In general, the pearls with a white nucleus had an obvious reflection band from 300nm to 400nm in the ultraviolet-visible absorption spectra. On the contrary, those pearls with a light or heavy gray nucleus had a reflection band from 400nm to 500nm. CONCLUSIONS The two types of gray pearls are all marine cultured pearls. The difference of ultraviolet absorption spectra of the two types of pearls can be used as a basis for judging whether the gray pearls are irradiatedly colored.

Keywords

• gray pearl
• ultraviolet-visible absorption spectroscopy
• structure
• nacreous layer
• nucleus
• irradiation treatment