Refractive Index (RI) is the characteristic slowing of light as it travels through a given gem species or substance. The higher the RI, the greater brilliance and luster is possible. The fact that each gem has its own RI makes it necessary to facet each species at slightly different angles and makes it a useful characteristic in gem identification.
What is the Index of Refraction?
Light moves at its maximum possible velocity in the absence of matter. For example, in space light travels approximately 186,000 miles per second. But the presence of matter slows light down; or more specifically, the forces which bind atoms slow light down. The Index of Refraction for any substance is the ratio of the velocity of light in a vacuum to its velocity in a given substance. The higher the index, the slower the velocity. Thus, if we say that a particular crystal has an index of refraction of 2.000 we mean that light would pass through a vacuum at twice the velocity that it can go through that crystal.
Why do different substances have different refractive indices?
As previously pointed out, the velocity of light through any material is retarded by forces binding its atoms and molecules. Therefore, the degree to which a particular beam of light is slowed depends both upon the types of atoms present (the chemical composition) and the nature of the arrangement of those atoms.
For example, light travels in water at 3/4 of its velocity in a vacuum, so we assign a value of 4/3, or 1.333 as the index of refraction of water. But if we freeze that water into ice, a different arrangement of the molecules results and the ice has a refractive index of 1.313. Or, if we boil the water, yet another arrangement of the same molecules is produced, which retards the light very little, and the steam has a refractive index of 1.0002.
In the case of ice, water and steam, we have examples of three different states of matter; solid, liquid and gas. But, even when a single state of matter is considered, it must be remembered that the identical materials can often be arranged in different structures, which are reflected in the differing indices of refraction.
For example, common pure Quartz crystals have an average index of 1.548. But if the crystal is melted and then cooled down to room temperature, the resulting fused silica-quartz has an index of 1.456. Or the same silicon dioxide may also exist as tridymite, cristobalite, coesite or other differing crystal structures, each of which has a different refractive index, although all have the identical chemical composition.
Does light always travel through a given crystal at the same speed?
No. Most minerals crystallize in an orderly lattice work that has different types of inter-molecular bonds in different planes, much like the grain in wood. The strength of these bonds may be very different in different planes.
The form of Calcite known as Iceland Spar presents an outstanding example of the effect on refractive index in different optical directions, since a given crystal will have an index of 1.486 in one direction and 1.658 in another. Fortunately (except for Rutile) most minerals used as gems do not vary so widely in different directions.
What other factors affect the Refractive Index of a mineral?
Temperature of the specimen influences its index, but if measurements are made at a normal range of room temperatures, this effect is so small that it can be neglected.