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You are here: Physical > Refraction

Refraction in gemstones

Single Refraction

Perhaps the surest single method of distinguishing precious stones is to find out the refractive index of the material. To one not acquainted with the science of physics this calls for some explanation. The term refraction is used to describe the bending which light undergoes when it passes (at any angle but a right angle) from one transparent medium to another. For example, when light passes from air into water, its path is bent at the surface of the water and it takes a new direction within the water.


Figure 1

 


AB represents the path of light in the air and BC its path in the water.


While every gem stone refracts light which enters it from the air, each stone has its own definite ability to do this, and each differs from every other in the amount of bending which it can bring about under given conditions. The accurate determination of the amount of bending in a given case requires very finely constructed optical instruments and also a knowledge of how to apply a certain amount of mathematics. However, all this part of the work has already been done by competent scientists, and tables have been prepared by them, in which the values for each material are put down.


The Herbert-Smith Refractometer. There is an instrument called the Herbert-Smith refractometer, through the use of which anyone with a little practice can read the refractive index of any precious stone that is not too highly refractive. Standard TIR refractometers in use outside of labs usually have a hemicylinder of lead glass with an upper limit RI of 1.90; the highest liquid easily available is 1.81 which thus becomes the effective upper limit; this would exclude very few stones of importance, i. e., zircon, diamond, sphene, and demantoid garnet.


Figure 2: Refractometer

Very rapid and very accurate determinations of stones may be made, and refractometer use is not confined to unmounted stones, for any stone whose table facet can be applied to the surface of the lens in the instrument can be determined.


The Herbert Smith refractometer measures the critical angles by total internal reflection, using a glass hemisphere through which light is directed upwards and reflected from the mineral surface. The reflected beam produces an image in the observing telescope, where a graduated scale measures the critical angle. The refractive index is then calculated by Snell's law.


Double Refraction

While glass and a small number of precious stones (diamond, garnet, and spinel) bend light as was illustrated in Figure 1, practically all the other stones cause a beam of light on entering them to separate, and the path of the light in the stone becomes double, as shown in Figure 3.

This behavior is called double refraction. It may be used to distinguish those stones which are doubly refracting from those which are not. For example, in the case of a stone which is doubly refracting to a strong degree, such as a peridot (the lighter yellowish-green chrysolite is the same material and behaves similarly toward light), the separation of the light is so marked that the edges of the rear facets, as seen through the table, appear double when viewed through a lens. A zircon will also similarly separate light and its rear facets also appear double-lined as seen with a lens from the table of the stone. The rarer stones, sphene and epidote, likewise exhibit this property markedly. Some colorless zircons, when well cut,  resemble diamonds so closely that even an expert might be deceived, if caught off his guard, but this simple test of looking for the doubled lines at the back of the stone would alone serve to distinguish the two stones.


Figure 3: Double Refraction


In the case of most of the other doubly refracting stones the degree of separation is much less than in peridot and zircon, and it takes a well-trained and careful eye to detect the doubling of the lines. Here a very simple device will serve to assist the eye in determining whether a cut stone is singly or doubly refracting. Expose the stone to direct sunlight and hold an opaque white card a few inches from the stone, in the direction of the sun, so as to get the bright reflections from within the stone reflected onto the card.


If the material is singly refractive (as in the case of diamond, garnet, spinel, and glass), single images of each of the reflecting facets will appear on the card, but if doubly refracting - even if slightly so - double images will appear. When the stone is slightly moved, these pairs of reflections will travel together as pairs and not tend to separate. The space between the two members of each pair of reflections serves to give a rough idea of the degree of the double refraction of the material if compared with the space between members in the case of some other kind of stone held at the same distance from the card. Thus zircon separates the reflections widely. Aquamarine, which is feebly doubly refracting, separates them but slightly.


It will be seen at once that this is very easily applied test and one that requires no costly apparatus. It is also a sure test after a little practice. For example, if a gemstone looks like a fine emerald, but is suspected of being glass, all that is needed is to expose it in the sun, as above indicated. If real emerald, double images will be evident (very close together, because emerald is feebly doubly refracting). If glass, the images on the card will be single.


Similarly, ruby can be easily distinguished from even the finest garnet or ruby spinel, as the last two are singly refracting. So, too, are glass imitations of ruby and ruby doublets (which consist of glass and garnet). This test cannot injure the stone, it may be applied to mounted stones, and it is reliable. For stones of very deep color this test may fail for lack of sufficiently brilliant reflections. In such a case hold the card beyond the stone and let the sunlight shine through the stone onto the card, observing whether the spots of light are single or double.


The table below gives the necessary information as to which stones show double and which single refraction.


Table 1: Character of Refraction in the Principal Gems


Refraction Single:
Diamond
Garnet (all types)
Spinel
Opal
Glass
   
Refraction Double: Difference between highest and lowest refractive indices
Sphene .084
Zircon .053
Benitoite .047
Peridot or chrysolite .038
Epidote .031
Tourmaline .020
Kunzite .015
Ruby and sapphire .009
Topaz (precious) .009
Emerald and aquamarine .007
Chrysoberyl .007

Differences in Refraction Due to Crystal Form. The difference in behavior toward light of the singly and doubly refracting minerals depends upon the crystal structure of the mineral. All gems whose crystals belong in the cubic system are singly refracting in all directions: In the case of some other systems of crystals the material may be singly refracting in one or in two directions, but doubly refracting in other directions. No attention need be paid to these complications when using the sunlight-card method with a cut stone, as in such cases the light in its course within the stone will have crossed the material in two or more directions, and the separation and consequent doubling of image will be sure to result.