Determination of the optic axial angle in biaxial gemstones and its use in gemmology

摘要

From the very beginning of the use of the refractometer in identification of gemstones, gemmologists faced two problems: how to differentiate between biaxial gemstones with similar or overlapping RIs. how to distinguish a uniaxial and a biaxial stone in an orientation which produced one variable and one constant shadow edge. The use of the polarizing filter to solve these problems can be avoided in many cases by the use of the optic axial angle method. The most important feature of this method is that it is reliable. Also, whether or not the method can be applied is quick to determine. Determination of an optic axial angle can be made on a simple diagram and the list of biaxial gemstones and their optic axial angles are shown. Determination of the optic axial angle requires the same data as determination of the optic sign but it is a much more discriminatory constant for use in identification of biaxial gemstones.