Читать книгу A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public - Frank Bertram Wade - Страница 14

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Cause of Color in Minerals. In Lesson III. we saw that many gem materials cause light that enters them to divide and take two paths within the material. Now all transparent materials absorb light more or less; that is, they stop part of it, perhaps converting it into heat, and less light emerges than entered the stone. If light of all the rainbow colors (red, orange, yellow, green, blue, violet) is equally absorbed, so that there is the same relative amount of each in the light that comes out as in the light that went into a stone, we say that the stone is a white stone; that is, it is not a colored stone. If, however, only blue light succeeds in getting through, the rest of the white light that entered being absorbed within, we say that we have a blue stone.

Similarly, the color of any transparent material depends upon its relative degree of absorption of each of the colors in white light. That color which emerges most successfully gives its name to the color of the stone. Thus a ruby is red because red light succeeds in passing through the material much better than light of any other color.

Unequal Absorption Causes Dichroism. All that has been said so far applies equally well to both singly and doubly refracting materials, but in the latter sort it is frequently the case, in those directions in which light always divides, that the absorption is not equal in the two beams of light (one is called the ordinary ray and the other the extraordinary ray).

For example, in the case of a crystal of ruby, if white light starts to cross the crystal, it not only divides into an ordinary ray and an extraordinary ray, but the absorption is different in the two cases, and the two rays emerge of different shades of red. With most rubies one ray emerges purplish red, the other yellowish red.

It will at once be seen that if the human eye could distinguish between the two rays, we would have here a splendid method of determining many precious stones. Unfortunately, the eye does not analyze light, but rather blends the effect so that the unaided eye gives but a poor means of telling whether or not a stone exhibits twin colors, or dichroism, as it is called. (The term signifies two colors.) A well-trained eye can, however, by viewing a stone in several different positions, note the difference in shade of color caused by the differential absorption.

The Dichroscope. Now, thanks to the scientific workers, there has been devised a relatively simple and comparatively inexpensive instrument called the dichroscope, which enables one to tell almost at a glance whether a stone is or is not dichroic. The construction is indicated in the accompanying drawing and description.

The Dichroscope.

If the observer looks through the lens (A) toward a bright light, as, for example, the sky, he apparently sees two square holes, Fig. 4.

Fig. 3. A, simple lens; B, piece of Iceland spar with glass prisms on ends to square them up; C, square hole.

Fig. 4.

What has happened is that the light passing through the square hole (C of Fig. 3) has divided in passing through the strongly doubly refracting Iceland spar (B of Fig. 3) and two images of the square hole are thus produced.

If now a stone that exhibits dichroism is held in front of the square hole and viewed toward the light, two images of the stone are seen, one due to its ordinary ray (which, as was said above, will have one color), and the other due to its extraordinary ray (which will have a different color or shade of color), thus the color of the two squares will be different.

With a singly refracting mineral, or with glass, or with a doubly refracting mineral when viewed in certain directions of the crystal (which do not yield double refraction) the colors will be alike in the two squares. Thus to determine whether a red stone is or is not a ruby (it might be a garnet or glass or a doublet, all of which are singly refracting and hence can show no dichroism), hold the stone before the hole in the dichroscope and note whether or not it produces twin colors. If there seems to be no difference of shade turn the stone about, as it may have accidentally been placed so that it was viewed along its direction of single refraction. If there is still no dichroism it is not a ruby. (Note.—Scientific rubies exhibit dichroism as well as natural ones, so this test will not distinguish them.)

A dichroscope may be had for from seven to ten dollars, according to the make, and everyone who deals in colored stones should own and use one.

Not all stones that are doubly refracting exhibit dichroism. White stones of course cannot exhibit it even though doubly refracting, and some colored stones, though strongly doubly refracting, do not exhibit any noticeable dichroism. The zircon, for example, is strongly doubly refracting, but shows hardly any dichroism.

The test is most useful for emerald, ruby, sapphire, tourmaline, kunzite and alexandrite, all of which show marked dichroism.

It is of little use to give here the twin colors in each case as the shades differ with different specimens, according to their depth and type of color. The deeper tinted stones of any species show the effect more markedly than the lighter ones.

The method is rapid and easy—it can be applied to mounted stones as well as to loose ones, and it cannot injure a stone. The student should, if possible, obtain the use of a dichroscope and practice with it on all sorts of stones. He should especially become expert in distinguishing between rubies, sapphires, and emeralds, and their imitations. The only imitation (scientific rubies and sapphires are not here classed as imitations), which is at all likely to deceive one who knows how to use the dichroscope is the emerald triplet, made with real (but pale) beryl above and below, with a thin strip of green glass between. As beryl is doubly refracting to a small degree, and dichroic, one might perhaps be deceived by such an imitation if not careful. However, the amount of dichroism would be less in such a case than in a true emerald of as deep a color.

Those who wish to study further the subject of dichroism should see Gem-Stones, by G. F. Herbert-Smith, Chapter VII., pp. 53–59, or see A Handbook of Precious Stones, by M. D. Rothschild, Putnam's, pp. 14–16.