The arithmetical difference between the refractive index of the ordinary ray (Omega) and that of the extra-ordinary ray (Epsilon) in the case of uniaxial minerals and that between the alpha and gamma in the case of Biaxial, at their maximum divergence, is known as the birefringence or double refraction. The following examples illustrate the above principle.
|Optic Character||Gemstone||Refractive Index||Birefringence (D.R.)|
|Anisotropic||Corundum||1.762 – 1.770||0.008|
|Anisotropic||Peridot||1.654 – 1.690||0.036|
- This is a property characteristic of anisotropic gemstones.
- The optic axis is a direction of single refraction hence no birefringence is seen in this direction.
A stone with higher birefringence will exhibit doubling. Inclusions, facet edges or an object viewed through the stone will appear double. A good illustration of double refraction occurs in transparent calcite. An object observed through an anisotropic substance will show double edges or a double image.