Abstract: The ornamental value and commercial value of gemstone can be greatly improved with the color change of natural gemstone by irradiation. But the gemstone irradiated by the nuclear reactor contains residual radioactive nuclides, which produce radiation exposure to the human. In this paper, the direct radioactivity measurement of irradiated gemstone was made by means of gamma ray spectrum analysis. The influence of geometric structure and density of irradiated gemstone on the accuracy of gamma ray spectrum measurement was analyzed by the method of experiment and Monte Carlo simulation. The calculated formula for the detection efficiency with energy and sample mass is obtained, and the accurate measurement of radioactivity of irradiated gemstone without a grinding process is realized.