Physical Characteristics of Precious Stones: Optical Properties

OPTICAL PROPERTIES:

Refraction.--When a luminous ray passes through a homogeneous medium, its course is in a straight line, as shown in Fig. 16, a phenomenon with which everyone is familiar. But when it passes from one medium into another, the case is generally different, and the ray suffers a remarkable modification. It is then more or less diverted from its primitive direction, and has the appearance of being broken, whence the phenomenon has been termed refraction. A stick plunged into water will exemplify this effect.

The extent to which the luminous rays are diverted in traversing transparent bodies varies greatly. This variation is generally connected with differences in the nature and composition of the refracting bodies; but it is likewise intimately connected, as experiments prove, with the molecular constitution of these bodies. For example, Iceland spar and aragonite, whose chemical composition is identical, both consisting of pure carbonate of lime, refract the light unequally, for the sole reason that their molecular constitution is very different.

Double Refraction.--Among diaphanous bodies there is a numerous class of substances that possess the curious quality of presenting two images of one object. If a crystal of Iceland spar is placed upon a piece of white paper bearing an inscription, as in Fig. 17, two images will be visible of every point, and both images will show deviation. This is an instance of what is called double refraction.

When a body, crystallized or not, is perfectly homogeneous in all its parts, so that its elements are disposed everywhere in a uniform manner, one can easily understand that the light must traverse it regularly, and must present a single image of every object: such bodies possess the property of simple refraction.

Crystals belonging to the monometric or tesseral system, as the cube or octahedron, since their molecular disposition is perfectly regular, never exhibit the phenomenon of double refraction, in whatever direction they are traversed by the light; but crystals of all other systems possess the power of double refraction, differing in its effects as the crystal appertains to a system more or less closely related to the regular system.

As all precious stones that are highly valued are crystallized, and it has been well ascertained what stones display simple and what stones double refraction, it sometimes becomes a matter of importance to know whether a given stone is really doubly-refrangent, in order to distinguish it from one which possesses simple refraction, but to which in other respects it is quite similar in appearance. As precious stones are all small, a special method of procedure is here necessary in order to obtain the phenomenon of double refraction.

Take, for example, a small stone, cut in form of a brilliant, concerning whose nature there is some doubt.

Place the stone at a level with the eye, holding it in one hand; in the other hand take an object of small dimensions, a pin for instance, and move it slowly on the other side of the stone until the eye is able to perceive it. If the stone is doubly-refractive, the rays will bifurcate on entering it, and accordingly two images of the pin will be seen, if it is not held too near the stone. If it is held very near the stone, the rays will not be far enough apart at the point where they emerge into the air to allow their separation to be evident.

If the experiment is made at night, instead of a pin a lighted candle may be used, the candle being placed beyond the reach of currents of air, so that its flame may be pure and regular. The phenomenon will be exactly the same, and have the aspect presented by Fig. 18.

If the phenomenon of double refraction is produced, the conclusion may be made without hesitation that the stone tested is not a diamond; for the diamond, since it appertains to the cubic (monometric) system, possesses simple refraction. The stone experimented on, therefore, is no doubt one of those with which the diamond is sometimes confounded, such as the sapphire and the zircon, which possess double refraction.

Polarization.--It is well known that if a beam of light falls upon a plane and polished surface it is reflected; but it is not so well known that if to this ray, which has already been reflected at a certain angle, a second mirror with plane inclined is presented, there are certain positions in which the ray will be no longer reflected by the second mirror. The light has acquired by its first reflection a profound modification, which is designated by the name of polarization by reflection.

In traversing certain crystals the light is subjected to the same changes, that is to say, the rays emerging from the crystal are no longer reflected when they fall at a certain angle upon a plane mirror; and they have become completely powerless to traverse certain crystals, otherwise perfectly translucid, when the latter are presented to them according to a determinate direction. The phenomenon thus presented is polarization by refraction.

Double refraction and polarization are qualities of crystals which are most intimately connected; and the combinations of these two manifestations produce magnificent phenomena of colouring, unattainable by substances producing simple refraction. It is very easy, by aid of a polariscope, to be assured upon the instant whether a precious stone possesses or not the power of double refraction.