Identification of An Unknown Variegated Bracelet

Shell is a kind of gem materials.There are many kinds of shells, which are usually cut and treated to different ornaments.Therefore it is difficult to identify their original shapes.Recently, a special kind of bracelet was found in the market.It is made of fourteen rectangular beads, which have four kinds of colour: Purple, orange, pink and white.It is opaque and has a laminated structure.The pink and orange part of bracelet have silky luster, and the bracelet has many things in common with shells.Therefore the study made comparison of the bracelet and other shell jewelries, which included Monetaria moneta, Pinctada margaritifera, Pincdata maxima, Trochus maculatus Linnaeus, Pteria penguin, etc.The fundamental gemmological characteristics of the bracelet and the other shell jewelries were tested by refractometer, balance, etc, and the composition and spectral features were measured by X-ray fluorescence spectrometer (XRF), X-ray diffraction (XRD), Fourier transformation infrared spectrometer (FTIR) and Raman spectrometer.The structure of the samples were analyzed by scanning electron microscope (SEM).According to the test results, the refractive index of the samples are all between 1.530 and 1.685, and the average refractive index of the sample is 1.57.The average relative density of the sample is 2.57, which is a little lower than the other shell jewelries.It may be caused by the difference of composition and structure, or by the influence of the hole on it.They have similar chemical composition, and are all rich in Ca and Sr.They have completely different mineral composition and structure.The bracelet is mainly made of calcite and it has a lath-like structure.The calcite crystal was arranged radically and has a low relative density and silky luster.The shell jewelries in the market are mainly made of aragonite and have a brick-wall structure, which is almost from nacre.The aragonite crystal was in parallel arrangement, which has higher relative density than the unknown sample, and has pearl luster and iridescence.The bracelet has not been dyed by organic dyes, and its colour may be caused by carotenoids according to Raman spectrum.In conclusion, the bracelet could be a kind of calcite-type shell or made from the calcite layer of a shell.The orange and pink colour may be directly caused by carotenoids, and the purple colour may be caused by carotenoids as well as protein complexation.