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 the fundamentals (Read here for 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, its velocity is reduced and 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.
Now, I will not go into the complex formula of how the RI is calculated using angle of sine and cosine. Instead, a much more easier approach is to understand that the RI is the following ratio:
Refractive Index = Velocity of Light in Air / Velocity of Light In Medium
Refractive Index can be measured using a Refractometer (which will be cover in another post).
The RI of the following gemstones are:
Jadeite Jade – 1.66
Nephrite Jade – 1.60
Corundum – 1.762 – 1.77
Spinel – 1.718
Later we will see how a Refractometer can determine the RI of a gemstone, which is one of the most important test for a gemologist in the identification of a gemstone.