Gemmological and Mineralogical Characteristics of Multicolour Laminated Marble Jade

A new kind of ornamental stone in the domestic jewelry market is becoming popular in recent years, because of its similar appearance to the pork. In this paper, the mineral components and spectral characteristics of the ornamental stone samples were analyzed by conventional gemmological tests, micro-petrography and a variety of modern testing techniques. The results show that the ornamental stone is composed of white, yellow, brown, red and various other coloured bands, it has oil waxy luster, translucent to slightly transparent.The overall structure has a remarkable orientation. Micro-petrography and X-ray powder diffraction analysis show that the main mineral component is carbonate, accounting for more than 90%, including calcite and a small amount of magnesite. The secondary minerals are iron bearing marl such as iron oxide, accounting for about 5%-10%. In addition, there are argillaceous residues formed by recrystallization of very small amounts of calcite, with medium coarse-grained crystalloblastic structure as a whole. Under polarizing microscope, the characteristics of calcite metamorphism and recrystallization are obvious and the characteristics of lamellar bicrystal can be seen.The infrared spectra show characteristic infrared absorption peaks at 712, 875, 1 420 cm^-1, which are respectively attributed to the in-plane bending vibration ν₄, out-of-plane bending vibration ν₂, anti-symmetric stretching vibration ν₃ of CO₃^2-, and the vibration mode ν₄ indicates that the main component is calcite. Raman spectra have characteristic Raman shift peaks at 1 086, 713 cm^-1 and 281 cm^-1, which are attributed to the out-of-plane bending vibration, in-plane bending vibration and lattice vibration of CO₃^2-. Both infrared spectra and Raman spectra indicate the vibration mode of carbonate groups.The chemical composition analysis show that in addition to CaCO₃, the ornamental stone samples also contain a variety of trace metal elements such as Al, Fe, Mn and Ti. Fe and Mn is the colour origin of the yellow, brown and red bands. The higher the content of Fe and Mn, the darker the body colour of the stone. Based on its UV-Vis spectral characteristics, it can be seen that Fe and Mn should be the main cause of the colour red, yellow and brown, and Mn and Ti have certain effect on the change of colour tone.In conclusion, the ornamental stone samples are calcium magnesium carbonate minerals containing a small amount of iron argillaceous matter.According to Gems-Nomenclature GB/T 16552-2017, it should be named as marble jade or carbonate jade.