We propose a robust method that can quantitatively discriminate genuine pearls from imitation ones by introducing the concept of entropy in the polarization-sensitive optical coherence tomography (PS-OCT). Qualitatively, by examining the birefringence properties of the nacre region of pearls with PS-OCT, the genuine pearls can be easily discriminated. To quantify the amount of birefringence formation, however, the concept of phase retardation entropy is introduced, which is expected to have a higher value when a PS-OCT tomogram has more diverse phase retardation values in its histogram. Experimental confirmation demonstrated that the phase retardation entropy of a genuine pearl was always higher than an imitated pearl. By experimenting with various genuine and imitation pearls, we can say that the phase retardation entropy is effective as a quantitative criterion for discriminating and evaluating pearls.