Diamonds And Luxury

An important optical property of many precious stones is that known as dichroism or pleochroism. A stone possessing this property, when observed in different directions will show different colours or shades of colour which may resemble each other more or less closely, or may, on the other hand, differ considerably. A mineral sometimes used as a cut stone, and known as " water-sapphire,-" exhibits this phenomenon to such a marked degree that it has received the name dichroite, although at the present time it is usually known to miner¬alogists as cordierite. A crystal of this mineral, when viewed in three particular directions, perpendicular to each other, appears of three distinct colours, namely, a fine dark blue, light blue, and greyish-yellow. In intermediate directions are seen intermediate tints, which approach one or other of the three principal colours according to the direction of the line of view. The three particular directions along which these maximum differences in colour are observable are definitely related to the crystalline form of the mineral; they are in fact the three crystallographic axes of the rhombic crystal.
The difference in colour shown by cordierite when viewed in different directions is very-great, but it is perhaps even greater in some kinds of tourmaline. In this mineral the colour, as seen in different directions through a crystal, varies from yellowish-brown to asparagus-green, or in other crystals (of the same mineral) between dark violet-brown and greenish-blue, or again in others between purple-red and blue, &c. Some dichroic precious stones show only very small differences in colour when viewed in different directions; the Yellowish-green chrysolite is an example. Indeed the majority of pale coloured stones are only feebly dichroic, stronger dichroism being exhibited by minerals having a deeper tone of colour.

Finally there are other minerals, such as garnet and spinel, which show no differences of colour in different directions ; these precious stones behave in this respect like their glass imitations.

Taking refraction as the basis of classification, we have seen that alhminerals can be divided into two groups, namely, those which possess single refraction and those Princess Cut Diamond Stud Earrings which possess double refraction. The former group will include those minerals which are not possessed of diehroism, while all dichroic minerals fall into the latter. Thus amorphous substances and those which crystallise in the cubic system are characterised by single refrac¬tion and absence of diehroism, while all coloured minerals included in the remaining five crystal systems are dichroic, and all without exception are doubly refracting.

The phenomenon of diehroism then, furnishes us with additional aid in distinguishing singly from doubly refracting stones. A body showing this character to even the feeblest degree cannot be amorphous nor can it be a cubic mineral. The apparent absence of diehroism however must be considered only as negative evidence in favour of single refrac¬tion, since diehroism may be present but so feeble as to be detected only with difficulty. It-has been shown above that the phenomena of double and of single refraction enables us to distinguish a ruby from a red spinel, and this is made still more easy from the fact that the hexagonal ruby is distinctly dichroic, while the cubic spinel does not possess this property. In the same way an imitation ruby of red glass could not be confused with the genuine stone, since the former, being amorphous, is not dichroic and shows the same colour in all directions.
The detection of diehroism usually requires the use of a special instrument. The most convenient instrument for the purpose is that devised by the Viennese mineralogist Haidinger, and known as a dichroscope. This instrument is inexpensive and easily used, and should be in the hand of every one who buys or sells precious stones, since a single glance through it is sufficient to establish the presence or absence of diehroism in a stone.

This instrument is shown in section, and of its actual size, in Fig. 28. It consists essentially of a cleavage rhombohedron, C, of Iceland-spar (calcite), which is longer in one direction than in others. At each of its oblique ends is cemented a glass prism or wedge, K, the outer surfaces of which are perpendicular to the long edge of the calcite rhombohedron. A brass tube, h, encloses these essential portions of the instrument, and has at one end a small square aperture, b, and at the other a circular aperture, a. Between the circular aperture and the glass prism, K, is placed a lens, L, of such a focal length that on looking through the instrument in the direction ab a sharp image of the square aperture, b, will be seen. This image will, however, owing to the intervention of the doubly refracting calcite, not be single, but double. The instrument is so proportioned that these two images, o and e, will appear side by side, in contact but not overlapping. The image o will be only slightly displaced from the axis of the instrument and will be quite colourless; the image e is rather more displaced, and shows a narrow red border on its inner edge and a narrow blue border on its outer edge, as indicated by striations in the small figure at the side, otherwise the image is colourless. The instrument is so constructed that the distance of the square aperture from the lens can be varied, which enables the images to be sharply focused and adjusted so that their edges are in contact.

In using this instrument, the precious stone to be tested for diehroism is placed over the square aperture, b, and, the instrument being directed towards a clear sky, the observer places his eye close to the round aperture, a. An object-carrier, H, is sometimes provided for the purpose of more conveniently holding the stone. This has the form of a brass tube fitting loosely over the tube, h, and having the closed end perforated by an aperture somewhat larger than the square aperture, b, over which the stone can be fixed with wax, as shown in the figure. This arrangement allows the carrier H, with the stone attached, to be rotated while the calcite rhombohedron remains unmoved. Should this carrier not be provided, the stone may be fixed by wax to a glass-plate, or simply held in the fingers in front of the square aperture, and the instrument rotated in the hand.

If the stone under examination is not dichroic, the two images o and e will be of the same colour and will show no variation while the instrument or the stone is rotated through 360°. If, for example, a red garnet, which crystallises in the cubic system, is examined, the two images o and e will both be of the same red colour as is the garnet itself when viewed without the aid of the instrument.
Check this also three stone rings.The images o and e given by a dichroic stone, on the contrary, will be in general differently coloured. In four particular positions, however, at 90° apart, the colours of the two images are identical. On rotating the stone or the instrument a difference between the colours appears, which gradually increases and reaches a maximum at 45° from the original position. Further rotation will again result in a gradual decrease in the colour difference, and at 90° from the original position the colours of the two images once more become iden¬tical. The same changes occur during the rotation through the remaining quadrants, and thus a complete rotation of 360° is accompanied by eight changes from identity of colour in the two images to maximum difference in colour between them and vice versd. The juxtaposition of the two images makes it possible to detect the smallest differences in colour, and consequently the slightest degree of dichroism.

We have previously seen that doubly refracting crystals are singly refracting along the direction of an optic axis;. similarly dichroic crystals exhibit no dichroism in these directions. To prove the absence of dichroism in a crystal it is therefore necessary to examine it in two directions, or, as an additional precaution, in a third direction also. After each observation the stone must be fixed on the holder, H, in a new position and again rotated. The absence of dichroism can be conclusively proved only after an examination of the stone in at least three different positions. The dichroism of a stone may be so feeble that it is not possible to detect it even with the aid of a dichroscope; moreover, it must be borne in mind that a coloured doubly refracting stone is not necessarily dichroic, and this feature is naturally absent in colourless doubly refracting stones. The real or apparent absence of dichroism in a stone is therefore no proof of its singly refracting character, but the presence of dichroism is, on the contrary, a conclusive proof of the doubly refracting nature of the stone.

The degree of dichroism in a crystal varies according to the direction through which the crystal is observed. The colours of the two images seen in the dichroscope in the examination of all dichroic stones become the more nearly identical as the optic axis of the stone becomes more nearly coincident with the axis of the instrument. Conversely, the greater the angle between the axis of the dichroscope and the optic axis of the stone the more marked will become the difference in colour between the two images. The two colours between which there is the maximum difference are known as the principal or axial colours ; these colours, as seen through the dichroscope, differ in tint from the colours the stone shows when observed with the naked eye in the same direction. Uniaxial dichroic crystals, such as tourmaline, show two principal colours, while biaxial crystals, such as cordierite, show three. The pairs of colours, other than the axial or principal colours, shown by a precious stone in the dichroscope are due to various combinations of the principal colours. The axial colours of each precious stone will be given below along with their special descriptions.

The detection of dichroism in a coloured stone is a simpler matter than the observation of double refraction, and the dichroscope is a less expensive instrument than the polariscope, hence the former is the more often used. The polariscope may also be used to detect the presence or absence of dichroism by removing the Nicol's prism, n (Fig. 27), and placing the stone to be tested on the object-carrier. If the stone is not dichroic, as for instance spinel, there will be no change of colour as it is rotated with the object-carrier. If, on the contrary, a dichroic stone, such as ruby, is examined in this way, the colour will be seen to change as the stone is rotated, varying between two extremes ; the change from one extreme to the other will occur four times during a complete rotation of 360°. These two colours are identical with those seen when the stone is Examined in the same position in a dichroscope ; the advantage of the latter instrument lies in the fact that the two colours can be seen side by side, and thus small differences between them more easily detected. Just as in the use of the dichroscope, several observations must be made before the absence of dichroism can be considered to have been conclusively proved. As explained before, in using the polariscope the portion of the field occupied by the stone may remain dark owing to the total reflection of light within the stone. This can be avoided as before, by placing the stone in a certain position or immersing it in a strongly refracting liquid. Care must also be taken that all side light, which might be reflected from the surface of the stone, is screened off with the hand, or by means of a paper tube placed around the stone.
As we have already seen, dichroism is a character of which important use can be made in identifying precious stones, and in distinguishing them from each other and from glass imitations. Moreover, its observation does not necessitate a mounted stone being removed from its setting, which would often be necessary in the observation of other optical characters.

Dichroism is a character of precious stones which is important also from other points of view, such as that of the lapidary. A stone in which dichroism is strong must be so cut that the rays of light received by the observer have passed through the stone in a direction such that they will appear of the finest colour possible. Such a stone as cordierite, for example, must be so cut that the dark blue colour is prominently brought out, which will give a far more pleasing effect than if the light blue or yellowish-grey were predominant. The beauty and, consequently, the value of two pleochroic stones of the same size and quality will accordingly depend upon the manner in which they are cut, and hence a knowledge of the dichroic properties of stones is of importance to the gem-cutter.
Dichroic stones are sometimes cut and mounted in a manner which will bring out this character as prominently as possible. With this object in view a cube is fashioned out of the stone, the faces of which are perpendicular to the directions in which the greatest differences in colour are exhibited. Such cubes are pivoted at one corner, so that on being turned round the different colours will successively come into view. Cordierite, andalusite, and other stones are cut and mounted in this way, as will be explained in more detail later.