Birefringence – What is it?

Abstract: Birefringence as explained in simple lay man language.

When a light ray passes from air to another medium, such as a gemstone, the light ray is bent towards or away from the normal inside the medium. This is called refraction.

The ratio of the speed of light in air to its speed in the gemstone is known as the Refractive Index (RI) of the gemstone.  RI is measured using a Refractometer.

The RI of a gemstone provides one of the most important piece of information for its identification.

Single Refractive (SR) refers to the instance where one single ray of incident light gives rise to one single ray of refractive light. Double Refractive (DR) refers to the instance where one single ray of incident light gives rise to two rays of refractive light.

Hence, the optical property of most gemstones are classified into Single Refractive (SR) or Double Refractive (DR).

A Single Refractive gemstone has a single RI reading, while a Double Refractive gemstone thus will have two RI readings.

Birefringence is the difference between the upper and the lower RI reading of a Double Refractive gemstone.

The best known SR gemstones are diamond, spinel and garnet. The RI of diamond is 2.417, which is beyond the reading on a normal gemological Refractomer. Spinel has a reading of 1.718 while garnet has a reading of 1.888.

Group of garnets showing the broad color range in the species and varieties. See related asset for collection numbers and identification of each garnet.

The best known DR gemstones are Corundum (rubies and sapphires), Beryl (emerald and aquamarine) and Peridot.

The RI and Birefringence of Corundum is 1.762 – 1.770 and .008 respectively.

We shall see in one of my later post on how to use birefringence to differentiate between a ruby and a spinel, as both of their color is red.

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