What is the refractive index in mineralogy ?
Refractive index : definition
This parameter largely determines the luster of minerals : a vitreous or related luster indicates an index between 1.3 and 1.9, an adamantine luster an index between 1.9 and 2.6, a sub-metallic luster an index between 2.6 and 3 and finally a metallic luster, an index greater than 3.
In the majority of minerals (except all those belonging to the cubic system and therefore isotropic), the refractive index varies according to the direction of propagation : these minerals are said to be birefringent, the incident ray splitting into 2 refracted rays. This property is the basis of crystal optics and the study of minerals by microscopy.