Abstract: A "moonstone" ring sent by the customer was encountered in the daily inspection. The appearance of the gem is colourless and has hazy moonlight effect. Through naked eye observation, it is very similar to the ordinary moonstone. Conventional gemmological identification and infrared spectrum analysis test confirmed that it is composed of alkyd resin, with artificial resin characteristics and characteristics of the infrared spectrum absorption peak. It is rare to use alkyd resin to imitate moonstone, which is easy to neglect, therefore, more attention should be paid to routine testing.