Gem minerals belong to two optical classes – Isotropic and Anisotropic
- Isotropic or Single Refracting (S.R.): A ray of light which enters an Isotropic substance will obey Snell’s lawof refraction and will produce a single refracted ray, regardless of the direction in which the ray enters the gem.
Isotropic or Single Refracting Gemstones:
- Amorphous materials (glass, opal, amber which lack a regular internal structure), as well as gemstones belonging to the Cubic system (diamond, garnet, spinel, fluorite etc.)
- They are all defined by a single refractive index, since light is propagated with equal velocity in all directions.
- The index of refraction of such optically isotropic crystals does not depend on the direction in which the ray of light passes through them.
- Anisotropic or Doubly Refracting (D.R.): A ray of light which enters an anisotropic substance is not only bent or refracted, but it is also broken or split into two rays, each of which travels with a different velocity and hence has a different Refractive Index.
- Each of these rays is polarised and differs from the other in velocity and vibration direction. Their vibration planes are perpendicular to each other.
- The light ray with a constant velocity of propagation in different directions is called the ordinary ray. The R.I. of the ordinary ray is constant i.e. it does not vary with direction of propagation.
- The rays of variable velocity of propagation of light in different directions are called the extra-ordinary rays. The R.I. of the extra-ordinary ray varies not only with direction of propagation but also with planes of polarization.
- Gemstones belonging to the tetragonal, hexagonal and trigonal systems have two principal R.I.‘s – Omega for ordinary ray and Epsilon for extra-ordinary ray.
- Gemstones belonging to orthorhombic, monoclinic and triclinic systems have three principal R.I.‘s – Alpha (least) for the fastest ray, Beta (intermediate) and Gamma (greatest) for the slowest ray.
- In addition, every day ray of light is differently refracted for different colours of light.
- A.D.R. (S.R.) = Anomalous Double Refraction (seen in S.R. stones)
- A.G.G. = Aggregate