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 seperate rays. This phenomenon 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 therefor polarised. Both these rays travel at different speeds inside the gemstone and thus will refact at different angles.
The strenght of DR (Double Refraction) varies with direction and we measure the maximum DR (Δ).
These maximum vaules differ from one gemstone to the other. 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 it's angle of refraction), this ray is name the ordinary ray (usually indicated with ω). The other will vibrate in the direction on 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 "double refraction" and is often indicated by "Δ" (greek letter delta). This maximum birefringence is largest when light enters the gemstone at an angle perpendicular to the optic axis.
A strong birefingence may result in visual doubling of facets (when the stone is facetted) as can be seen in a large numbers 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.
Gemstones belonging to the cubic crystal system and amorphous gems have only one RI and therefor do not show birefringence, all other gemstones do.
Uniaxial stones (those crystallizing in the trigonal, hexagonal and hexagonal systems) will show two readings and have one optic axis.
Biaxial gemstones (Orthorhombic, monoclinic and triclinic systems) have twp directions in which the incident light will react as it were isotropic and therefor have two optic axes.