Abstract: The gemmological characteristics of ordinary Tridacna and petrified Tridacna have been invetigated by conventional gemmological testing instruments, X-ray powder diffraction spectrometer, Fourier transform infrared spectrometer, Raman spectrometer, polarizing microscope, and scanning electron microscope. The authors try to find out the detailed different points between ordinary Tridacna and petrified Tridacna by analogy.The results of basic gemmological tests show that petrified Tridacna has higher transparency, higher relative density, higher Mohs hardness and stronger ultraviolet fluorescence.X-ray powder diffraction analysis reveals that the major mineral component of both petrified Tridacna and ordinary Tridacna are aragonite.The IR spectrum and Raman spectrum of the two kinds of Tridacnas both have aragonite characteristic peaks.Under the influence of internal trace elements, the peaks of v₂ and v₃ show different degree of blue shift compared with inorganic aragonite.The internal structure of the samples are observed by polarizing microscope and scanning electron microscope(SEM). The results show that the internal aragonite grains of petrified Tridacna are in oriented arrangement. petrified Tridacna has compact internal structure with parallel layered structure as the characteristic structure. Ordinary Tridacna has parallel layered structure and crossed lamellar structure, whose internal structure is messy and loose.