Cause of Colour in Minerals
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 colours (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 coloured 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 colour of any transparent material depends upon its relative degree of absorption of each of the colours in white light. That colour which emerges most successfully gives its name to the colour of the stone. Thus a ruby is red because red light succeeds in passing through the material much better than light of any other colour.
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 that, in the 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 analyse 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 colours, or dichroism, as it is called. (The term signifies two colours.) A well-trained eye can, however, by viewing a stone in several different positions, note the difference in shade of colour caused by the differential absorption.
The Dichroscope. A relatively simple and comparatively inexpensive instrument called the dichroscope (Figure 1) enables the determination of whether a stone is or is not dichroic. The construction is indicated in the Figure 2 drawing and accompanying description. The dichroscope consists of a simple lens (A), a piece of Iceland spar with glass prisms on ends to square them up (B), and a square hole (C).
If the observer looks through the lens (A) toward a bright light, as for example, the sky, he apparently sees two square holes (E). What has happened is that the light passing through the square hole (C) has divided in passing through the strongly doubly refracting Iceland spar (B) and two images of the square hole are thus produced.
If 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 colour), and the other due to its extraordinary ray (which will have a different colour or shade of colour), thus the colour 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 colours 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 colours. 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: Laboratory grown rubies exhibit dichroism as well as natural ones, so this test will not distinguish them.)
A dichroscope is an inexpensive instrument, 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 coloured 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 detail 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.