Refractive Index

It has already been explained how minerals refract or bend a beam of light. As the light hits the flat surface at an angle, it bends upon entering the gem. If the light beam's direction is slowly changed so that it comes to the surface of the gem at an increasingly lower angle, eventually a point is reached where it ceases to bend sufficiently to enter the gem. It just grazes the surface. Any further lowering of the beam causes it to be totally reflected away from the gem. The grazing angle is called the critical angle, and it will differ with each gem substance according to its refracting ability.

An instrument, the gem refractometer, has been devised to measure this critical angle quickly and easily. The instrument usually contains a built-in scale from which the refracting ability, or "refractive index," of the gem can be read directly. To read the index on a typical gem refractometer, of which there are several models on the market, one of the polished faces of the gem is placed against a polished piece of very highly refracting glass mounted in the instrument. Good contact is assured by placing a drop of highly refracting liquid between them. A light beam is brought through the glass to the gem. Any of the light coming to the gem from an angle at which it will be refracted is bent into the gem, away from the instrument, and is lost. Light coming in at an angle beyond the critical is reflected back into the instrument, hits the viewing eyepiece, and its trace shows as a bright section on the scale. The numerical marking on the scale dividing the light portion—representing reflected light—and the dark portion—representing the lost light refracted into the gem—is the critical angle. For convenience, this is numbered on the scale as the refractive index. The measurement is sufficiently precise so that, by consulting a table listing the refractive indices of gemstones, one can usually make a quick identification of the gem in question.

There are other methods for determining refractive index that are somewhat less convenient. One of the best of these uses a series of fluids of known refractive indices. When a gem is placed in a liquid having the same refractive index it effectively disappears. This happens because the gem does not cause any further bending of light than that already done by the liquid. We have no visual evidence of the gem's presence because it behaves toward light exactly as the liquid does. And also round diamond earrings An appropriate series of test liquids such as clove oil (index 1.54) to test for quartz (index 1.54), cassia oil (index 1.60) to test for topaz (index 1.61) and methylene iodide (index 1.74) to test for spinel (index 1.72-1.73) is easily assembled. Although some of the liquids are expensive and difficult to obtain and may be unstable enough to need frequent replacement, the method has some benefits for example diamond solitare earrings. It can be used with very small gem-stones fragments, and the flat, polished surfaces required for the refractometer are not necessary. Also, even when the gem material disappears in a liquid of appropriate index, the cracks, flaws, and foreign inclusions in it do not vanish. This affords an excellent method of checking a gemstone internally to see how it can best be cut to make the most perfect gem. It also may make it easier to see and study the nature of the inclusions.