When light strikes a cut or polished stone, one or more of the following effects are observed:–it may be transmitted through the stone, diaphaneity, as it is called; it may produce single or double refraction, or polarisation; if reflected, it may produce lustre or colour; or it may produce phosphorescence; so that light may be (1) transmitted; (2) reflected; or produce (3) phosphorescence.

To the quality of refraction is due one of the chief charms of certain precious stones. It is not necessary to explain here what refraction is, for everyone will be familiar with the refractive property of a light-beam when passing through a medium denser than atmospheric air. It will be quite sufficient to say that all the rays are not equal in refractive power in all substances, so that the middle of the spectrum is generally selected as the mean for indexing purposes.

It will be seen that the stones in the 1st, or cubic system, show single refraction, whereas those of all other systems show double refraction; thus, light, in passing through their substance, is deviated, part of it going one way, the other portion going in another direction–that is, at a slightly different angle–so that this property alone will isolate readily all gems belonging to the 1st system.

A well-known simple experiment in physics shows this clearly. A mark on a card or paper is viewed through a piece of double-refracting spar (Iceland spar or clear calcite), when the mark is doubled and two appear. On rotating this rhomb of spar, one of these marks is seen to revolve round the other, which remains stationary, the moving mark passing further from the centre in places. When the spar is cut and used in a certain direction, we see but one mark, and such a position is called its optical axis.

Polarisation is when certain crystals possessing double refraction have the power of changing light, giving it the appearance of poles which have different properties, and the polariscope is an instrument in which are placed pieces of double-refracting (Iceland) spar, so that all light passing through will be polarised.

Since only crystals possessing the property of double refraction show polarisation, it follows that those of the 1st, or cubic system–in which the diamond stands a prominent example–fail to become polarised, so that when such a stone is placed in the polariscope and rotated, it fails _at every point_ to transmit light, which a double-refracting gem allows to pass except when its optical axis is placed in the axis of the polariscope, but this will be dealt with more fully when the methods of testing the stones come to be considered.

Diaphaneity or the power of transmitting light:–some rather fine trade distinctions are drawn between the stones in this class, technical distinctions made specially for purposes of classification, thus:–a “non-diaphanous” stone is one which is quite opaque, no light of any kind passing through its substance; a “diaphanous” stone is one which is altogether transparent; “semi-diaphanous” means one not altogether transparent, and sometimes called “sub-transparent.” A “translucent” stone is one in which, though light passes through its substance, sight is not possible through it; whilst in a “sub-translucent” stone, light passes through it, but only in a small degree.

The second physical property of light is seen in those stones which owe their beauty or value to REFLECTION: this again may be dependent on Lustre, or Colour.

Lustre This is an important characteristic due to reflection, and of which there are six varieties:–([alpha]) adamantine (which some authorities, experts and merchants subdivide as detailed below); ([beta]) pearly; ([gamma]) silky; ([delta]) resinous; ([epsilon]) vitreous; ([zeta]) metallic. These may be described:–

([alpha]) Adamantine, or the peculiar lustre of the diamond, so called from the lustre of adamantine spar, which is a form of corundum (as is emery) with a diamond-like lustre, the hard powder of which is used in polishing diamonds. It is almost pure anhydrous alumina (Al_{2}O_{3}) and is, roughly, four times as heavy as water. The lustre of this is the true “adamantine,” or diamond, brilliancy, and the other and impure divisions of this particular lustre are: _splendent_, when objects are reflected perfectly, but of a lower scale of perfection than the true “adamantine” standard, which is absolutely flawless. When still lower, and the reflection, though maybe fairly good, is somewhat “fuzzy,” or is confused or out of focus, it is then merely _shining_; when still less distinct, and no trace of actual reflection is possible (by which is meant that no object can be reproduced in any way to define it, as it could be defined in the reflection from still water or the surface of a mirror, even though imperfectly) the stone is then said to _glint_ or _glisten_. When too low in the scale even to glisten, merely showing a feeble lustre now and again as the light is reflected from its surface in points which vary with the angle of light, the stone is then said to be _glimmering_. Below this, the definitions of lustre do not go, as such stones are said to be _lustreless_.

([beta]) Pearly, as its name implies, is the lustre of a pearl.

([gamma]) Silky, possessing the sheen of silk, hence its name.

([delta]) Resinous, also explanatory in its name; amber and the like come in this variety.

([epsilon]) Vitreous. This also explains itself, being of the lustre of glass, quartz, etc.; some experts subdividing this for greater defining accuracy into the “sub-vitreous” or lower type, for all but perfect specimens.

([zeta]) Metallic or Sub-metallic. The former when the lustre is perfect as in gold; the latter when the stones possess the less true lustre of copper.

Colour. Colour is an effect entirely dependent upon light, for in the total absence of light, such as in black darkness, objects are altogether invisible to the normal human eye. In daylight, also, certain objects reflect so few vibrations of light, or none, that they appear grey, black, or jet-black; whilst those which reflect all the rays of which light is composed, and in the same number of vibrations, appear white. Between these two extremes of _none_ and _all_ we find a wonderful play and variety of colour, as some gems allow the red rays only to pass and therefore appear red; others allow the blue rays only and these appear blue, and so on, through all the shades, combinations and varieties of the colours of which light is composed, as revealed by the prism. But this is so important a matter that it demands a chapter to itself.

The third physical property of light, PHOSPHORESCENCE, is the property possessed by certain gems and minerals of becoming phosphorescent on being rubbed, or on having their temperature raised by this or other means.

It is difficult to say exactly whether this is due to the heat, the friction, or to electricity. Perhaps two or all of these may be the cause, for electricity is developed in some gems–such as the topaz–by heat, and heat by electricity, and phosphorescence developed by both.

For example, if we rub together some pulverised fluorspar in the dark, or raise its temperature by the direct application of heat, such as from a hot or warm iron, or a heated wire, we at once obtain excellent phosphorescence. Common quartz, rubbed against a second piece of the same quartz in the dark, becomes highly phosphorescent. Certain gems, also, when merely exposed to light–sunlight for preference–then taken into a darkened room, will glow for a short time. The diamond is one of the best examples of this kind of phosphorescence, for if exposed to sunlight for a while, then covered and rapidly taken into black darkness, it will emit a curious phosphorescent glow for from one to ten seconds; the purer the stone, the longer, clearer and brighter the result.

Many stones are capable of exhibiting the same phenomenon, not only by friction, as in Thales’s experiment, but also under the influence of light, heat, magnetism, chemical action, pressure, etc., and of holding or retaining this induced or added power for a long or short period, according to conditions and environment.

If a small pith ball is suspended from a non-conducting support, it forms a simple and ready means of testing the electricity in a stone. According to whether the ball is repelled or attracted, so is the electricity in the stone made evident, though the electroscope gives the better results.

By either of these methods it will be found that some of the stones are more capable of giving and receiving charges of electricity than are others; also that some are charged throughout with one kind only, either positive or negative, whilst others have both, becoming polarised electrically, having one portion of their substance negative, the other positive. For instance, amber, as is well known, produces negative electricity under the influence of friction, but in almost all cut stones, other than amber, the electricity produced by the same means is positive, whereas in the uncut stones the electricity is negative, with the exception of the diamond, in which the electricity is positive.

When heated, some stones lose their electricity; others develop it, others have it reversed, the positive becoming negative and vice versâ; others again, when heated, become powerfully magnetic and assume strong polarity.

When electricity develops under the influence of heat, or is in any way connected with a rising or falling of temperature in a body, it is called “pyro-electricity,” from the Greek word “pyros,” fire. The phenomenon was first discovered in the tourmaline, and it is observed, speaking broadly, only in those minerals which are hemimorphic, that is, where the crystals have different planes or faces at their two ends, examples of which are seen in such crystals as those of axinite, boracite, smithsonite, topaz, etc., all of which are hemimorphic.

Taking the tourmaline as an example of the pyro-electric minerals, we find that when this is heated to between 50° F. and 300° F. it assumes electric polarity, becoming electrified positively at one end or pole and negatively at the opposite pole. If it is suspended on a silken thread from a glass rod or other non-conducting support in a similar manner to the pith ball, the tourmaline will be found to have become an excellent magnet.

By testing this continually as it cools there will soon be perceived a point which is of extreme delicacy of temperature, where the magnetic properties are almost in abeyance. But as the tourmaline cools yet further, though but a fraction of a degree, the magnetic properties change; the positive pole becomes the negative, the negative having changed to the positive.

It is also interesting to note that if the tourmaline is not warmed so high as to reach a temperature of 50° F., or is heated so strongly as to exceed more than a few degrees above 300° F., then these magnetic properties do not appear, as no polarity is present. This polarity, or the presence of positive and negative electricity in one stone, may be strikingly illustrated in a very simple manner: -If a little sulphur and red-lead, both in fine powder, are shaken up together in a paper or similar bag, the moderate friction of particle against particle electrifies both; one negatively, the other positively.

If, then, a little of this now golden-coloured mixture is gently dusted over the surface of the tourmaline or other stone possessing electric polarity, a most interesting change is at once apparent. The red-lead separates itself from the sulphur and adheres to the negative portion of the stone, whilst the separated sulphur is at once attracted to the positive end, so that the golden-coloured mixture becomes slowly transformed into its two separate components -the brilliant yellow sulphur, and the equally brilliant red-lead. These particles form in lines and waves around the respective poles in beautiful symmetry, their positions corresponding with the directions of the lines of magnetic force, exactly as will iron filings round the two poles of a magnet.

From this it will clearly be seen how simple a matter it is to isolate the topaz, tourmaline, and all the pyro-electric stones from the non-pyro-electric, for science has not as yet been able to give to spurious stones these same electric properties, however excellent some imitations may be in other respects.

Further, almost all minerals lose their electricity rapidly on exposure to atmospheric influences, even to dry air; the diamond retains it somewhat longer than most stones, though the sapphire, topaz, and a few others retain it almost as long again as the diamond, and these electric properties are some of the tests which are used in the examination of precious stones.

Those stones which show electricity on the application of pressure are such as the fluorspar, calcite, and topaz.

With regard to magnetism, the actual cause of this is not yet known with certainty. It is, of course, a self-evident fact that the magnetic iron ore, which is a form of peroxide, commonly known as magnetite, or lodestone, has the power of attracting a magnet when swinging free, or of being attracted by a magnet, to account for which many plausible reasons have been advanced. Perhaps the most reasonable and acceptable of these is that this material contains molecules which have half their substance positively and the other half negatively magnetised.

Substances so composed, of which magnets are an example, may be made the means of magnetising other substances by friction, without they themselves suffering any loss; but it is not all substances that will respond to the magnet. For instance, common iron pyrites, FeS2, is unresponsive, whilst the magnetic pyrites, which varies from 5FeS, Fe2S3, to 6FeS, Fe2S3, and is a sulphide of iron, is responsive both positively and negatively. Bismuth and antimony also are inactive, whilst almost all minerals containing even a small percentage of iron will deflect the magnetic needle, at least under the influence of heat.

So that from the lodestone -the most powerfully magnetic mineral known -to those minerals possessing no magnetic action whatever, we have a long, graduated scale, in which many of the precious stones appear, those containing iron in their composition being more or less responsive, as already mentioned, and that either in their normal state, or when heated, and always to an extent depending on the quantity or percentage of iron they contain.

In this case, also, science has not as yet been able to introduce into an artificial stone the requisite quantity of iron to bring it the same analytically as the gem it is supposed to represent, without completely spoiling the colour. So that the behaviour of a stone in the presence of a magnet, to the degree to which it should or should not respond, is one of the important tests of a genuine stone.