Abstract: In conventional gem identification methods, whether the emeralds turn red under the Chelsea filter can be used as an auxiliary evidence for emerald identification, but this identification standard has limitations. In order to more accurately describe the colour changes of emeralds under the Chelsea filter, this article applied the colorimetry method to convert the transmittance spectra that passed through the Charles filter into chromaticity coordinates. Thus, the colour changes of emeralds under Chelsea filter can be presented in the form of data. This article first studied the influence of testing angle of spectra, contact way between emerald and Chelsea filter, and model difference of Chelsea filters and other factors on the emerald's chromaticity under Chelsea filter. Then, the optical loss was analyzed during the process of passing through the Chelsea filter. Based on the optical loss, the modulation algorithm of the Chelsea filter for emeralds' spectra was established. The distribution of colour coordinates on the CIE 1931 chromaticity diagram corresponding to the natural emerald and synthetic emerald spectra modulated by Chelsea filter is identifiable. The location distribution of chromaticity coordinates can be used as a basis to distinguish natural emeralds from synthetic emeralds to a certain extent.