Gemmological Characteristics of Agate from Madagascar

The conventional gemmological methods,polarizing microscope,X-ray powder diffraction spectrometer,electron microprobe,infrared spectrometer,and cathodoluminescence were used in this paper for studying the agate sample from Madagascar with special white patterns.The results show that the chemical composition of the part with different colours in the agate sample from Madagascar is SiO₂,and the main mineral component is quartz.The quartz fiber parallels each other or intersects in small angle.On the basis of analysis of X-ray powder diffraction peaks at 68°,the crystallinity index of agate sample from Madagascar is 1.33,which is much lower than that of ideal quartz crystal(crystallinity index is 10).It is presumed that agate sample from Madagascar was formed by a fast siliciaous magmatic melting at the low temperature.