Double Refraction

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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 "birefringence".
The maximum birefringence of a gemstone is named "double refraction".


Basic

Incident ray separated into an ordinary ray (ω) and an extra-ordinary ray (ε)


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 "double refraction", often indicated by the symbol "Δ" (Greek letter delta). This maximum birefringence is largest when light enters the gemstone at an angle perpendicular to the optic axis.

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.

Related Topics

References

  • Gemmology 3rd edition (2005) - Peter G. Read