Abstract: The gem stone which is called Larimar occurs in the Chinese jewelry marketnow.The gemmological characteristics,chemical compositions,X-ray powder diffraction and infrared spectrum of the Larimar sample are analysed in the paper in order to define its mineral group.The results show that the Larimar sample is mainly composed of pectolite.The ultraviolet-visible absorption spectrum of the sample suggests that the absorption characteristic of the sample is related to Cu.It is supposed that the blue colour of Larimar probably could be caused by the diffusion of Cu-bearing sulfide distrubuted in pectolite according to its absorption spectrum and gemmological characteristics and chemical compositions.The definition of the mineral group of Larimar could be of significance to its identification and naming.