Characteristic and Cause of Colour of the Yellow and Black Rough Stone Crusts of Laos Stone from Water Area

The rough stone crusts of Laos stones from water area are mostly yellow or black, which have a certain similarity to the macroscopic features of one of the famous Chinese seal stone Tianhuang. Based on naked-eye observation and experiments using gemmological microscope, X-ray powder diffractometer, Fourier infrared spectrometer, scanning electron microscopes and laser Raman spectrometer, the mineral components of the rough stone crusts were analyzed and the cause of the yellow and black colour was studied. The results shown that the main mineral components of the stone crusts are dickite, kaolinite and their transitional minerals, and the minor minerals include pyrite, quartz, calcite, dolomite, svanbergite and titanium oxide. A large amount of amorphous carbon distributes on the black crust, and iron oxide fills along the cracks on the yellow crust. Therefore, the black colour crust of Laos stone from water area is related to the amorphous carbon distributed on the surface, and the yellow colour is related to iron oxide.