There are totally 18 kinds of refractivity models in all potential polished faces for gem crystals. In actual refractivity measurements, only 15 models are recognizable. For all anisotropic gems, a maximum index of refractivity, a minimum index of refractivity, and a birefringence can be determined directly from its corresponding refractivity model. The recognization of "groove-shape" on a face that intersects obliquely its z-axis in a uniaxial gem makes it possible to differentiate itself from a model of biaxial gem. As a result, we can undoubtedly distinguish an isotropic gem, a uniaxial gem and a biaxial gem. Because of the difference between the "groove-shape" model in an uniaxial gem and the "sawtooth-shape" model in a biaxial gem, we are able to directly determine optical sign from six kinds of refractivity modle that were so alike in pair that we weren't able to distinguish them before (Table 1: e and h; f and g; j and k). We still cannot determine optical sign in the following refractivity models: b, m, and o yet in Fig. 1.
展开▼
