The Refractive Index (RI) is one of the most important optical property of a gemstone. Once the RI of a gemstone is known, one can easily identify the gemstone, given another few tests to confirm its identity.
Let us go back to some fundamentals to understand what is Refractive Index.
When a light ray strikes a shiny surface part of its light ray is reflected back. Remember your science lesson that the Law of Reflection states that the angle of incidence is equal to the angle of reflectance and that both these rays are on the same plane.
When a light ray passes from air to another medium, such as a gemstone, the light ray is bent. Refraction is a commonly observed phenomenon in our everyday life. When a pencil is placed in a glass one will observe that the pencil appears bent. Or you will see a swimming pool is actually shallower than what you actually see because of refraction.
When a ray of light passes from air into another transparent or semi-transparent medium such as a gemstone, its speed is greatly reduced and the light ray is bent. The ratios of the speed of light in air to its speed in the gemstone is known as the Refractive Index (RI) of the medium.
The RI of the following gemstones are:
Jadeite Jade – 1.66
Nephrite Jade – 1.60
Corundum (ruby or sapphire) – 1.762 – 1.77
Refractive Index can be measured using a Refractometer.
Acknowledgement: onlinetuition.com.my, gia.edu, gemologyproject.com