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Some gemstones have more than one refractive index (RI) because these stones belong to crystal systems (anistropic) that have atomic structures which cause an incident ray of light to be resolved (split) into two separate rays. This phenomenon is named "double refraction".
The maximum double refraction of a gemstone is named "birefringence".
When a ray of light enters the gemstone, the atomic structure allows only those rays vibrating in two specific directions to continue.
These two rays vibrate in planes that are mutually perpendicular and are therefore polarized. Both these rays travel at different speeds inside the gemstone and thus will refract at different angles.
The strength of DR (Double Refraction) varies with direction and we measure the maximum DR (Δ).
These maximum values differ from one gemstone to another. For instance:
- Strong DR - zircon (0.059)
- Medium DR - tourmaline (0.020)
- Low DR - quartz (0.009)
In uniaxial gemstones, one ray will vibrate in the direction perpendicular to the optic axis and will obey Snell's Law (one can calculate its angle of refraction). This ray is named the ordinary ray (usually indicated with ω). The other ray will vibrate in the direction of the optic axis and does not obey Snell's Law (i.e. the angle of refraction will vary). That ray is named the extra-ordinary ray (indicated by ε).
The maximum RI difference between these two rays is named "birefringence", often indicated by the symbol "Δ" (Greek letter delta). This maximum double refraction is largest when light enters the gemstone at an angle perpendicular to the optic axis. When light enters the gem at an angle parallel to the optic axis, the birefringence will be 0 (zero).
Although in gemology the term "birefringence" usually indicates the maximum difference between the ordinary and extra-ordinary rays (or the α and γ readings in biaxial stones), the term is also applied to any variation in refractive indices. Not just the maximum.
In faceted stones, a strong birefringence may result in visual doubling of facets, which is observed in a large number of zircons. Although the DR is at its maximum when viewed in the direction perpendicular to the optic axis, no doubling of facets will be seen in that direction due to superimposition.
When an anistropic stone is examined in the direction parallel to an optic axis, the stone will behave as an istropic gemstone. Therefore no doubling of facets will be seen in that direction either.
Gemstones belonging to the cubic crystal system and amorphous gems have only one RI and therefore do not show birefringence; all other gemstones do.
Uniaxial stones (those crystallizing in the trigonal, hexagonal and tetragonal systems) will show two readings and have one optic axis.
Biaxial gemstones (orthorhombic, monoclinic and triclinic systems) have two directions in which the incident light will react as if it were isotropic and therefore will have two optic axes.
- Richard H. Cartier - A new definition of optic axes for gemmology and the four kinds of optic axes, The Journal of Gemmology, Vol. 29 no.4 (October 2004) pp. 228 - 234
- Gemmology 3rd edition (2005) - Peter G. Read