Abstract: Due to the nondestructive requirements of gem testing, evaluation of precious stones shows characteristics of science and uncertainty. Gemological application of X-ray Diffraction (XRD) is specialized, with its application mainly concentrated in three aspects:① Gemstone identification:phase identification of the monocrystalline gemstones and polycrystalline gemstones; origin of the gemstones; classification of the pleomorphic gem minerals. For example, according to the XRD patterns of microcrystalline cryptocrystalline quartz variants, different silica mineral phases of the chalcedony can be distinguished accurately; types of Di in bloodstone can be distinguished through the composition and content of clay mineral. ② Research on crystal structure of the gem minerals:analysis of crystal chemistry of the monocrystalline gems with complex composition; situation of the substitution of mineral isomorphism; quality of the gem by using the degree of crystallinity. For example, through the c/a ratio in lattice parameters of beryl, aluminium octahedral isomorphism and the beryllium tetrahedral isomorphism can be distinguished. The relationship between the crystallinity calculated from XRD data and the quality of nephrite is close. ③ Development of gemological research technology:to monitor the process of gemstone optimization; to inspect the mineral crystal size, crystalline quality and internal stress during the synthetic process of gemstones; to research the artificial breeding technology based on the crystal structure of organic gemstones. Take the heat treatment process of spinel as example:magnesium irons moved from octahedral M-O to tetrahedral T-O, resulting in the bond length of M-O and T-O changing upon temperature and being reflected in the spinel color. During the synthesis process of topaz, XRD data shows that the structure of the formed fluorine topaz is similar to natural topaz at 770-800℃. At 950℃ and 1000℃, the fluorine topaz is decomposed to corundum and mullite. XRD research in the gemological application began relatively recently in China. In other countries, application in gem identification, optimization and synthesis are relatively mature. There is still much room for the development of XRD when combined with other technologies such as the Scanning Electron Microscope and Proton Microprobe.