Colour Origin of Tahiti Black Pearl

Pearls are an organic-inorganic mixture secreted by shellfish. Black pearls from Tahiti, whose breeding process is complex and long are known for their unique dark black tone. In this study, the gemmological characteristics, chemical compositions and spectral characteristics of the black pearl samples from Tahiti were studied using infrared spectroscopy, ultraviolet-visible spectroscopy, laser Raman spectroscopy, XRF, XRD, and SEM, in order to clarify their colour causing and provide the pearl cultivation and identification. The research shows that the black pearl samples from Tahiti vary in shape, with body colour ranging from dark black to gray, with the diameter between 9.4 mm and 11.0 mm, and relative density between 2.667 and 2.749. Some black pearl samples from Tahiti are overtone, show darken red or yellow-green fluorescence under long wavelength UV light. Observed under the gem microscope, it is found that defects mainly are threads and circular pits, as well as the fingerprint-like texture. Infrared spectrum result shows that all samples had consistent absorption peaks at 707, 879, 1 483 cm^-1, which are in-plane bending vibration of aragonite O-C-O, the out-of-plane bending vibration of O-C-O, and antisymmetric stretching vibration of CO₃^2- ions, respectively (Fig. 1). The UV-Vis spectra of the black pearl samples have characteristic peaks at 280, 404, 493, 699 nm, among which 280, 699 nm are stronger in the peak intenty(Fig. 2) and negatively correlated with black tone. Laser Raman sepectrum result further verifies that the main mineral of black pearl sample is aragonite, with the characteristic peaks at 1 083, 703 cm^-1, respectively, corresponding to the symmetric stretching vibration of the carbonate ion and in-plane bending vibration of C-O-C. Organic pigment porphyrins make black pearl samples from Tahiti darker. XRF result shows that the black pearls are rich in various elements, but the contents of elements are not significantly related to pearl's colour. The ratio of Sr/Ca confirms that black pearl samples are seawater cultural pearls. Based on XRD analysis, black pearl is mainly composed of aragonite and a small amount of calcite. The cross section of black pearl reveals white or pale yellowish brown nucleus and black nacreous layer, and boundary between the nucleus and the layer is very clear through scanning electron microscopy. The nacreous layers are uniform colour, approximately 1 mm thick, similar to brick-wall structure, smooth and orderly packed. The terraced texture is very obvious on the surface of the pearl, and the aragonite tablets are closely arranged, regular and almost equal in size. The study led to the following conclusions: (1)black pearl samples from Tahiti are larger in diameter, with typical pearl luster and fingerprint-like texture; (2)the brick-wall structure of nacreous layer and the surface characteristics of nucleus are extremly important to affect the luster of the pearl; (3)the main mineral of black pearl sample from Tahiti is aragonite, and its colour causing is related to the content of calcite and the structure of the nacreous layer; (4)the body colour of black pearl sample from Tahiti is negatively correlated with the intensity of UV-Vis light peaks at specific wavelengths and positively correlated with organic porphyrin pigments.