The spectral types of usual gray pearls were classified based on UV-Vis diffuse reflection spectra. The coloring mechanism of gray pearl was further investigated. The results show that :(1) based on the presence or absence of absorption peak nearby 280 nm in the reflectance spectra of gray pearls, which are firstly divided into typeⅠ (obvious absorption) andⅡ (no absorption or weak absorption). Furthermore, according to the shape of spectra and the position of the main wavelength of the reflection peak, the type Ⅰ pearls are further divided into three subtypes: Ⅰn, Ⅰp andⅠf. Among them, the color of the nucleus of typeⅠp pearl is white, and a brown or blackish-brown transition layer exists between the nucleus and nacre, which may be the direct cause of the gray color of the pearl. Meanwhile, the type ofⅠp pearls are generally considered to be untreated in the field of gemstone identification. (2) because of the change in the color of nacre before and after irradiation and the variation characteristics of the UV-Vis reflection spectrum in this work and the previous work on irradiated pearls, it is preliminarily concluded that irradiation is still one of the main reasons for the coloring mechanism of gray pearls. Moreover, the pearl can be identified as having been treated based on the disappearance of the absorption peak or the presence of only one absorption shoulder near 280 nm in the corresponding reflectance spectrum. According to the typeⅡ gray pearls section structure, the nucleated and non-nucleated pearls coexist in the circulation field. A more accurate method used to identify the irradiated pearl should be developed.