Diamonds And Luxury

The fact that doubly refracting crystals are singly refracting along their optic axes must not be forgotten in making observations with the polariscope. A doubly refracting stone, placed in the instrument so that its optic axis coincides with the line, of vision, will behave as if it were singly refracting, and will remain dark during a complete rotation of the carrier. A single observation of this kind is therefore not sufficient to prove the singly refracting character of the stone.

The probability of a stone being placed in the instrument in this position will, as a rule, be small; when a stone gives the indications of single refraction, explained above, it should be placed on the carrier in another position and re-examined. bridal sets. A second indication of the same kind may be regarded as conclusive, though in case this second indication should be due to the second optic axis coinciding with the line of vision— a very improbable chance—the stone may be examined in a third position. As a rule, an examination of the stone in two positions will be sufficient to establish its singly refracting character. If, on the other hand, a stone should at the first trial give the indications of a doubly refracting substance, the observation may be regarded as conclusive and further examination is superfluous.

When a stone in the polariscope gives the appearances peculiar to singly refracting substances after examination in two, or even in three, positions, it has been stated above that it may legitimately be concluded that the stone is really singly refracting. We have now to show that, under certain circumstances, such is not the case, and that the stone may be in reality doubly refracting. When a cut stone is examined in the polariscope, the facets on the side turned towards the observer will not be parallel to the facet upon which the stone lies, but they may be very steeply inclined to it. Light travelling vertically upwards from beneath will always be able to enter the stone by the facet on which it lies ; it may, however, strike the upper, steeply inclined facets so obliquely as to be totally reflected and pass out at the sides of the stone, thus never reaching the eye of the observer. The error which this may lead to may be avoided in several ways.

The majority of cut stones have, as shown in Plates II.-IV., a large facet, the table, on one side, and a small facet, the culet, parallel to the table, on the opposite side. If the stone be examined through these two parallel faces, there will be no possibility of internal total reflection. With this object in view, the stone should be placed on the object-carrier so as to rest upon the culet, the table being uppermost; should the culet be very small the stone may be supported by pieces of wax. With the stone in this position, the light entering it will strike the table perpendicularly and there will thus be no chance of reflection from this facet. Moreover, the position in which the stone rests upon the culet has the further advantage that the whole area of the table is available for the egress of the light which enters the stone by the culet; whereas, in the reverse position, much of the light which enters by the table will fail to escape by the culet, but will be totally reflected from the side facets.

When a stone, examined in the polariscope in the position just described, gives the indications of a singly refracting substance, this observation, as explained above, cannot be regarded as conclusive, and the stone must be re-examined in another position. In any other position, however, there is a possibility of a doubly refracting stone appearing to be singly refracting owing to total reflection of the light within it.

2ct diamond engagement ring. This possibility may be avoided by the following simple device:
The stone is completely immersed in a strongly refracting liquid contained in a small glass vessel placed on the object-carrier. The difference between the index of refraction of the stone and of the surrounding medium will be much less than when the stone was in air, and the result will be, as has already been explained, that a larger proportion of the light will escape total reflection and will pass out of the stone. The amount of light totally reflected will in any case be considerably diminished, but total reflection will not be entirely absent, unless the refractive index of the liquid is exactly the same as that of the stone. In the case of diamond, however, this state of affairs will not exist, since there is no liquid with so high a refractive index.

Liquids used for this purpose should be transparent, not deeply coloured, and of high refractive index. One which fulfils these conditions, and has been already mentioned, is methylene iodide. It is one of the most strongly refracting liquids known, having at the ordinary temperature of 15° to 20° C. an index of refraction of 1"75 for the middle rays of the spectrum ; this value for the refractive index is exceeded by only few precious stones, notably the diamond, the index of refraction of which is 2'43. From the upper faces of a feebly refracting stone immersed in methylene iodide there will be no total reflection of light, but this will still take place if a diamond is substituted for the feebly refracting stone. All rays of light, forming with the normal to the surface from which they emerge an angle greater than 46° 19' (Fig. 13); in other words, all rays travelling vertically upwards will be totally reflected from facets inclined to the horizontal object-carrier of the polariscope at an angle greater than 46° 19'. Total reflection in diamond is thus not eliminated but considerably diminished in amount, the corresponding critical angle for diamond in air being 24° 24'.
A drawback to the use of methylene iodide for this purpose is its high price; monobromonaphthalene is a much cheaper liquid, with a refractive index almost as high as that of methylene iodide; it can therefore be used as a substitute for the latter in optical determinations, but not in determinations of specific gravity, being too light a liquid for this purpose.

It will be well at this point to review the method of using the polariscope for determining the singly or doubly refracting character of a precious stone. diamond engagement ring. The stone is placed on the object-carrier in the dark field of the polariscope and the carrier rotated. If now, all side light being carefully screened off, the field shows alternations of lightness and darkness the stone may be considered to be without doubt doubly refracting. Should the field remain dark during the rotation, the stone must be placed in another position on the object-carrier and again rotated. If this rotation results in alternations of lightness and darkness in the field, the stone is certainly doubly refracting; but should the whole of the field still remain dark we cannot conclude that the stone is singly refracting until it has been proved that the absence of light has not been due to total reflection within the stone. With this object the stone must be examined for the third time, either in the position described above, resting upon the culet and with the table horizontal and uppermost, or immersed in a strongly refracting liquid to diminish or eliminate total reflection. If on rotation the whole of the field still remains dark the stone must be singly refracting. For the examination of stones cut in a spherical form, or those with a rough and irregular surface, it will often be necessary to immerse them in liquid at first. Observations made with the polariscope require no special skill, and with practice and attention to the necessary precautions are very reliable.

Before leaving the subject of refraction certain anomalous cases must be considered. Many singly refracting substances, such, for example, as diamond, occasionally show the appearances peculiar to doubly refracting substances. When this is the case, such substances are said to possess anomalous double refraction. The phenomenon is frequently due to internal strains set up on the solidification of the substance or brought about by subsequent causes. These internal strains may be so great in certain crystals, for example those diamonds known as " smoky stones," as to cause such stones to fly to pieces without any apparent reason.
Anomalous double refraction is usually, however, only feeble, and the alternations of lightness and darkness exhibited in the polariscope by minerals exhibiting this character are much less marked than is the case with truly doubly refracting stones. Moreover, the illumination of a doubly refracting body is uniform over the whole of its area; but in the case of a body showing anomalous double refraction, portions of its area will remain dark during the complete rotation of the carrier, while the portions which become alternately light and dark during the rotation appear as stripes and bands, or in variously shaped sectors. It is thus a comparatively easy matter to distinguish between anomalous and true double refraction.

The phenomenon now under consideration sometimes affords a means whereby a glass imitation may be distinguished from a genuine precious stone. While glass under ordinary conditions is singly refracting, being an isotropic substance, unannealed glass possesses anomalous double refraction. Thus if a fairly thick plate of glass be first strongly heated and then suddenly cooled, internal strains will be set up, and when examined in the polariscope it will show a more or less regular black cross, with the two arms at right angles, sometimes surrounded by coloured circles. Should a supposed precious stone show this or a similar appearance, the observer may regard it as conclusive evidence against the genuineness of the stone.

The refraction of a precious stone is expressed, as explained above, by a number known as its refractive index. In the case of singly refracting substances there is, for monochromatic light, only one refractive index, which is constant for every direction in the stone. The index of refraction of a doubly refracting substance, however, varies according to the direction. It is greatest in one particular direction and least in a direction at right angles to this. These maximum and minimum values vary slightly for differently coloured light, but are constant for monochromatic light. The greater the difference between the greatest and the least values of the refractive indices of a precious stone, the greater will be its double refraction, which is measured by this difference.

The number which thus expresses the strength of the double refraction of a substance is constant for, and characteristic of, that substance, and could be made use of for purposes of identification. Its exact determination is, however, a matter of considerable difficulty and requires special and costly instruments, as well as suitable preparation of the stone to be examined. This method is therefore of no practical value to jewellers.
The refractive indices of the more important precious stones are given in the following table, the values for singly refracting stones being indicated by n, and the greatest and least values for doubly refracting stones by ng and nz respectively. In both cases the values apply to the middle rays of the spectrum. The strength of the double refraction of each stone is indicated by d = n — nt, that is, by the difference between the greatest and the least refractive indices of the stone.