Refraction of light
Diamond, like all other substances, which crystallize in the cubic system, is singly refracting. A ray of light incident obliquely upon the plane face of a diamond is propagated in the substance of the stone as a single ray, the direction of which, however, differs from that of its path in the surrounding medium. This difference is, in diamond, very considerable, much more than in the majority of other substances; in other words, the index of refraction of diamond is very high.
The dispersion coefficient is thus: 2.46476 - 2.40735 = 0.05741.
For comparison, the following values of the refractive indices of a particular glass may be given:
The dispersion coefficient is here less than half as great as that of diamond.
1.544684 - 1.524312 = 0.020372,
Under the same conditions, the prism of diamond will produce a spectrum over twice as long as a spectrum produced by a prism of glass.
Anomalous double refraction
Diamond, being crystal listed in the cubic system, should be singly refracting, that is, isotropic. This, however, is only strictly true for such stones as are perfectly colorless, or of a yellowish color, and are quite free from enclosures of foreign matter, cracks, and other flaws. Such faultless stones when rotated in the dark field of the polarized light remain dark. As previously mentioned, the stone under examination should be immersed in methylene iodide, so as to diminish total reflection as far as possible.
Deeply colored stones, and those disfigured by cracks, enclosures, or other faults, when placed in the dark field of the polarized light, allow the passage of light to the eye, but as a rule, to only a small extent. They have, under these circumstances, a grayish appearance, brilliant polarization colors being rarely seen. The feeble double refraction possessed by such stones is not an essential character of the substance of the diamond itself, but is due to disturbing influences; hence it is distinguished as anomalous double refraction. During its rotation in the polarized light it rarely happens that such a stone is uniformly dark or uniformly light over its whole surface; as a rule, certain areas are dark while others are light, and vice versa. Frequently certain regularly bounded areas or fields behave in a similar manner during rotation, while adjacent fields behave differently. In most cases, however, the areas showing these differences in behavior have no definite arrangement relative to each other, and areas showing a feeble double refraction are often enclosed in areas, which are perfectly isotropic.
The doubly refracting portions of the stone usually surround enclosures or cracks, and it is in the immediate vicinity of these that double refraction is strongest and the polarization colors most brilliant. As the distance from a flaw of this kind increases the double refraction becomes feebler, and at a certain distance disappears. Sometimes a black cross, the arms of which consist of two dark brushes, is seen when a stone is examined in the polarized light the arms of the cross are mutually perpendicular and their point of intersection coincides with an enclosure in the diamond. It is clear that such an appearance is due to a strain in the diamond brought about by the presence of the enclosure, and that the strain will be less in portions further removed from the enclosure.
Although the anomalous double refraction of diamond is, as rule, but feeble, stones exist in which it is comparatively strong, and which show much brighter polarization colors. This is the case in the "smoky stones" of South Africa, which, because of the great internal strain in their substance, have a tendency to fall to powder for no apparent reason. A parallel case is that of the drops of glass known as "Prince Rupert's drops," which also show strong double refraction as a consequence of internal strain.
There is never the slightest danger of confusing anomalous with true double refraction, for a mineral with true double refraction, such for example as quartz, colorless sapphire or topaz, will appear much more brilliantly illuminated when examined in the polarized light, and, moreover, will be uniformly light or uniformly dark over its whole surface.
Diamond is often regarded as the type of what a perfectly clear, colorless, and transparent stone should be. It can by no means, however, be always so regarded, since cloudy and opaque diamonds are actually more common than those, which are clear and transparent, while very great variety in color is found in this mineral. A great number of diamonds are indeed perfectly colorless, and correspond strictly to the popular conception of the stone; this number is, however, only one-fourth of the total number of diamonds found; another quarter show a very light shade of color, while the remainder, at least one-half of the total, are more or less deeply colored.
Perfectly colorless diamonds are, at the same time, freest from impurity. Absolutely pure carbon, crystallized in the form of diamond, shows no trace of color whatever, and stones of this purity are naturally highly prized. A peculiar steel-blue appearance is sometimes observed in stones, which combine absence of color with perfect transparency. With the exception of a few especially beautifully colored stones of great rarity, these blue-ni1e diamonds are the most highly prized of all; they are not of great rarity in India and Brazil, but occur in South Africa with far less frequency.
Any coloring matter intermixed with the substance of a diamond imparts its color to the stone, the tone of which will he faint when the pigment is present in small amount and deeper when it is present in greater amount. In all cases the amount of coloring matter relative to the mass of the stone is extremely small.
Investigations into the precise nature of the various coloring matters present in diamonds have seldom been undertaken on account of their difficulty and expense. There can be no doubt, however, that the coloring matter of many diamonds is of an organic nature, possibly some one or other of the hydrocarbons; in other cases the pigment is probably inorganic material in an extremely fine state of division. We have already seen that colored diamonds contain a small amount of ferruginous material, which remains behind as an incombustible ash after the diamond is burnt away, and that with colorless diamonds this is not the case. There seems sufficient grounds here for the inference that in such cases the color of the stone is due to the inorganic, incombustible, enclosed material, especially as the color is neither altered nor destroyed after exposure to high temperature, which would he the case were it organic in nature.
Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting Diamond trade
Rafal Swiecki, geological engineer email contact
This document is in the public domain.