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DIAMOND

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 power of breaking up white light into its constituent colors, that is, the dispersion, possessed by diamond is likewise very marked. The blue rays of light undergo a much greater refraction when passing into diamond than do the red rays; hence the spectrum produced by a prism of diamond is very long, the red and blue ends being widely separated. The various colors into which white light, in passing through a cut diamond, is broken up are widely separated and distinctly perceptible; hence the beautiful play of brilliant, prismatic colors upon which so much of the beauty of diamond depends, and which differentiates it so markedly from other colorless stones, such as rock-crystal, topaz, colorless sapphire, etc. which have a lower dispersion and consequently a less beautiful play of colors. This subject has, however, been fully dealt with above in the section devoted to the consideration of the passage of light through a cut stone.
The action of every diamond upon light is not absolutely identical. A satisfactory explanation of the small differences which exist cannot, however, at present be given. The fact that one stone bas a finer appearance than another is probably due to slight differences between them in the refractive index and in dispersive power. In respect of play of prismatic colors Indian diamonds rank highest. Next to these we may place Brazilian stones from the district of Diamantina in the State of Minas Geraes, and from the Canavieiras mines in the State of Bahia. Relatively inferior to these, but yet with a fine play of prismatic colors, are the majority of Cape diamonds. It is a remarkable fact that in many cases diamonds from the Cape and from Canavieiras exhibit a finer play of prismatic colors in artificial light than in daylight, which is the reverse of what is usually the case.
The refractive power and the dispersion of diamond are both given by the values of the refractive indices for different colored rays. These values give the strength of refraction directly, and the difference between the refractive index for red and that for violet rays is a measure of the dispersion. A comparison of the following determinations by Walter, with similar constants for other precious stones, will show that the refraction and dispersion of diamond are in all cases the greater.

Red light (B line of the spectrum)   2.40735
Yellow light (D line of the spectrum)   2.41734
Green light (E line of the spectrum)   2.42694
Violet light (H line of the spectrum)   2.46476

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:

Red light   1.524312
Yellow   1.527982
Green   1.531372
Violet   1.544684

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.

Color

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.

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This document is in the public domain.

March, 2011