There are, however, certain minerals which possess distinctive features in their qualities of hardness, colour, transparency, refractability or double refractability to light-beams, which qualities place them in an entirely different class to the minerals of a metallic nature. These particular and non-metallic minerals, therefore, because of their comparative rarity, rise pre-eminently above other minerals, and become actually “precious.”

This is, at the same time, but a comparative term, for it will readily be understood that in the case of a sudden flooding of the market with one class of stone, even if it should be one hitherto rare and precious, there would be an equally sudden drop in the intrinsic value of the jewel to such an extent as perhaps to wipe it out of the category of precious stones. For instance, rubies were discovered long before diamonds; then when diamonds were found these were considered much more valuable till their abundance made them common, and they became of little account. Rubies again asserted their position as chief of all precious stones in value, and in many biblical references rubies are quoted as being the symbol of the very acme of wealth, such as in Proverbs, chapter iii., verses 13 and 15, where there are the passages, “happy is the man that findeth wisdom … she is more precious than rubies”–and this, notwithstanding the enormous quantity of them at that time obtained from the ruby mines of Ophir and Nubia, which were then the chief sources of wealth.

It will also be remembered that Josephus relates how, at the fall of Jerusalem, the spoil of gold was so great that Syria was inundated with it, and the value of gold there quickly dropped to one-half; other historians, also, speaking of this time, record such a glut of gold, silver, and jewels in Syria, as made them of little value, which state continued for some considerable period, till the untold wealth became ruthlessly and wastefully scattered, when the normal values slowly reasserted themselves.

Amongst so many varieties of these precious minerals, it cannot be otherwise than that there should be important differences in their various characteristics, though for a stone to have the slightest claim to be classed as “precious” it must conform to several at least of the following requirements:–It must withstand the action of light without deterioration of its beauty, lustre, or substance, and it must be of sufficient hardness to retain its form, purity and lustre under the actions of warmth, reasonable wear, and the dust which falls upon it during use; it must not be subject to chemical change, decomposition, disintegration, or other alteration of its substance under exposure to atmospheric air; otherwise it is useless for all practical purposes of adornment or ornamentation.

There are certain other characteristics of these curious minerals which may be classified briefly, thus:–Some stones owe their beauty to a wonderful play of colour or fire, due to the action of light, quite apart from the colour of the stone itself, and of this series the opal may be taken as a type. In others, this splendid play of colour is altogether absent, the colour being associated with the stone itself, in its substance, the charm lying entirely in the superb transparency, the ruby being taken as an example of this class of stone. Others, again, have not only colour, but transparency and lustre, as in the coloured diamonds, whilst the commoner well-known diamonds are extremely rich in transparency and lustre, the play of light alone showing a considerable amount of brilliancy and beauty of colour, though the stone itself is clear. Still others are opaque, or semi-opaque, or practically free from play of light and from lustre, owing their value and beauty entirely to their richness of colour.

In all cases the value of the stone cannot be appreciated fully till the gem is separated from its matrix and polished, and in some cases, such as in that of the diamond, cut in variously shaped facets, on and amongst which the light rays have power to play; other stones, such as the opal, turquoise and the like, are cut or ground in flat, dome-shaped, or other form, and then merely polished. It frequently happens that only a small portion of even a large stone is of supreme value or purity, the cutter often retaining as his perquisite the smaller pieces and waste. These, if too small for setting, are ground into powder and used to cut and polish other stones.

Broadly speaking, the greatest claim which a stone can possess in order to be classed as precious is its rarity. To this may be added public opinion, which is led for better or worse by the fashion of the moment. For if the comparatively common amethyst should chance to be made extraordinarily conspicuous by some society leader, it would at once step from its humbler position as semi-precious, and rise to the nobler classification of a truly precious stone, by reason of the demand created for it, which would, in all probability, absorb the available stock to rarity; and this despite the more entrancing beauty of the now rarer stones.

The study of this section of mineralogy is one of intense interest, and by understanding the nature, environment, chemical composition and the properties of the stones, possibility of fraud is altogether precluded, and there is induced in the mind–even of those with whom the study of precious stones has no part commercially–an intelligent interest in the sight or association of what might otherwise excite no more than a mere glance of admiration or curiosity. There is scarcely any form of matter, be it liquid, solid, or gaseous, but has yielded or is now yielding up its secrets with more or less freedom to the scientist. By his method of synthesis (which is the scientific name for putting substances together in order to form new compounds out of their union) or of analysis (the decomposing of bodies so as to divide or separate them into substances of less complexity), particularly the latter, he slowly and surely breaks down the substances undergoing examination into their various constituents, reducing these still further till no more reduction is possible, and he arrives at their elements. From their behaviour during the many and varied processes through which they have passed he finds out, with unerring accuracy, the exact proportions of their composition, and, in many cases, the cause of their origin.

It may be thought that, knowing all this, it is strange that man does not himself manufacture these rare gems, such as the diamond, but so far he has only succeeded in making a few of microscopic size, altogether useless except as scientific curiosities. The manner in which these minute gems and spurious stones are manufactured, and the methods by which they may readily be distinguished from real, will be dealt with in due course.

The natural stones represent the slow chemical action of water, decay, and association with, or near, other chemical substances or elements, combined with the action of millions of years of time, and the unceasing enormous pressure during that time of thousands, perhaps millions, of tons of earth, rock, and the like, subjected, for a certain portion at least of that period, to extremes of heat or cold, all of which determine the nature of the gem. So that only in the earth itself, under strictly natural conditions, can these rare substances be found at all in any workable size; therefore they must be sought after assiduously, with more or less speculative risk.

At the very commencement we are faced with a subject on which mineralogists and geologists are by no means in full agreement, and there seems just ground for considerable divergence of opinion, according to the line of argument taken. It is a most remarkable fact that, precious as are certain stones, they do not (with a few exceptions) contain any of the rarer metals, such as platinum, gold, etc., or any of their compounds, but are composed entirely of the common elements and their derivatives, especially of those elements contained in the upper crust of the earth, and this notwithstanding the fact that gems are often found deep down in the earth.

This is very significant, and points to the conclusion that these stones were formed by the slow percolation of water from the surface through the deeper parts of the earth, carrying with it, in solution or suspension, the chemical constituents of the earth’s upper crust; time and long-continued pressure, combined with heat or cold, or perhaps both in turn, doing the rest, as already mentioned.

The moisture falling in dew and rain becomes acidulated with carbonic acid, CO_{2} (carbon dioxide), from the combustion and decay of organic matter, vegetation, and other sources, and this moisture is capable of dissolving certain calcareous substances, which it takes deep into the earth, till the time comes when it enters perhaps a division-plane in some rock, or some such cavity, and is unable to get away. The hollow becomes filled with water, which is slowly more and more charged with the salts brought down, till saturated; then super-saturated, so that the salts become precipitated, or perhaps crystallised out, maybe by the presence of more or other salts, or by a change in temperature. These crystals then become packed hard by further supplies and pressure, till eventually, after the lapse of ages, a natural gem is found, _exactly filling_ the cavity, and is a precious find in many cases.

If now we try to find its analogy in chemistry, and for a moment consider the curious behaviour of some well-known salts, under different conditions of temperature, what is taking place underground ceases to be mysterious and becomes readily intelligible.

Perhaps the best salt for the purpose, and one easy to obtain for experiment, is the sulphate of sodium–known also as Glauber’s Salt.

It is in large, colourless prisms, which may soon be dissolved in about three parts of water, so long as the water does not exceed 60° F., and at this temperature a super-saturated solution may easily be made. But if the water is heated the salt then becomes more and more insoluble as the temperature increases, till it is completely insoluble.

If a super-saturated solution of this Glauber’s Salt is made in a glass, at ordinary atmospheric temperature, and into this cold solution, without heating, is dropped a small crystal of the same salt, there will be caused a rise in temperature, and the whole will then crystallise out quite suddenly; the water will be absorbed, and the whole will solidify into a mass which exactly fits the inner contour of the vessel.

We have now formed what _might_ be a precious stone, and no doubt would be, if continuous pressure could be applied to it for perhaps a few thousand years; at any rate, the formation of a natural jewel is not greatly different, and after being subjected for a period, extending to ages, to the washings of moisture, the contact of its containing bed (its later matrix), the action of the changes in the temperature of the earth in its vicinity, it emerges by volcanic eruption, earthquake, landslip and the like, or is discovered as a rare and valuable specimen of some simple compound of earth-crust and water, as simple as Glauber’s Salt, or as the pure crystallized carbon.

It is also curious to note that in some cases the stones have not been caused by aqueous deposit in an already existing hollow, but the aqueous infusion has acted on a portion of the rock on which it rested, absorbing the rock, and, as it were, replacing it by its own substance. This is evidenced in cases where gems have been found encrusted on their matrix, which latter was being slowly transformed to the character of the jewel encrusted, or “scabbed” on it.

The character of the matrix is also in a great measure the cause of the variety of the stone, for it is obvious that the same salt-charged aqueous solution which undergoes change in and on ironstone would result in an entirely different product from that resting on or embedded in silica.

Following out the explanation of the aqueous solution, in which the earth-crust constituents are secreted, we find that the rarer and more precious metals do not generally enter into the composition of precious stones–which fact may advisedly be repeated. It is, of course, to be expected that beryllium will be found in the emerald, since it is under the species beryl, and zirconium in zircon; but such instances are the exception, and we may well wonder at the actions of the infinite powers of nature, when we reflect that the rarest, costliest and most beautiful of all precious stones are the simplest in their constituents.

Thus we find the diamond standing unique amongst all gems in being composed of one element only–carbon–being pure crystallised carbon; a different form from graphite, it is true, but, nevertheless, pure carbon and nothing else. Therefore, from its chemical, as well as from its commercial aspect, the diamond stands alone as the most important of gems.

The next in simplicity, whilst being the most costly of all, is the ruby, and with this may be classed the blue sapphire, seeing that their chemical constituents are exactly the same, the difference being one of colour only. These have two elements, oxygen and aluminium, which important constituents appear also in other stones, but this example is sufficient to prove their simplicity of origin.

Another unique stone is the turquoise, in that it is the only rare gem essentially containing a great proportion of water, which renders it easily liable to destruction, as we shall see later. It is a combination of alumina, water, and phosphoric acid, and is also unique in being the only known valuable stone containing a phosphate.

Turning to the silica series, we again find a number of gems with two elements only, silica–an important constituent of the earth’s crust–and oxygen–an important constituent of atmospheric air. In this group may be mentioned the opal, amethyst, agate, rock-crystal, and the like, as the best known examples, whilst oxygen appears also mostly in the form of oxides, in chrysoberyl, spinel, and the like. This silica group is extremely interesting, for in it, with the exception of the tourmaline and a few others, the composition of the gems is very simple, and we find in this group such stones as the chrysolite, several varieties of topaz, the garnet, emerald, etc., etc.

Malachite and similar stones are more ornamental than precious, though they come in the category of precious stones. These are the carbonate series, containing much carbonic acid, and, as may be expected, a considerable proportion of water in their composition, which water can, of course, be dispelled by the application of heat, but to the destruction of the stone.

From all this will be seen how strong is the theory of aqueous percolation, for, given time and pressure, water charged with earth-crust constituents appears to be the origin of the formation of all precious stones; and all the precious stones known have, when analysed, been found to be almost exclusively composed of upper-earth-crust constituents; the other compounds which certain stones contain may, in all cases, be traced to their matrix, or to their geological or mineralogical situation.

In contradistinction to this, the essentially underground liquids, with time and pressure, form metallic minerals and mineralise the rocks, instead of forming gems.

Thus we see that in a different class of minerals–compounds of metals with the sulphates, such as sulphuric acid and compounds; also those containing the metallic sulphides; in cases where the metalliferous ores or the metallic elements enter into composition with the halogens–bromine, chlorine, fluorine, and iodine–in all these, precious stones are comparatively common, but the stones of these groups are invariably those used for decorative or ornamental purposes, and true “gems” are entirely absent.

It would therefore appear that though metallic minerals, as already mentioned, are formed by the action of essentially _underground_ chemically-charged water–combined with ages of time and long-continued pressure, rocks and earth being transformed into metalliferous ores by the same means–precious stones (or that portion of them ranking as jewels or gems) must on the contrary be wholly, or almost wholly, composed of _upper_-earth-crust materials, carried deep down by water, and subjected to the action of the same time and pressure; the simpler the compound, the more perfect and important the result, as seen in the diamond, the ruby, and the like.

The difference between a break or fracture and a “cleave,” is that the former may be anywhere throughout the substance of the broken body, with an extremely remote chance of another fracture being identical in form, whereas in the latter, when a body is “cleaved,” the fractured part is more readily severed, and usually takes a similar if not an actually identical form in the divided surface of each piece severed.

Thus we find a piece of wood may be “broken” or “chopped” when fractured across the grain, no two fractured edges being alike; but, strictly speaking, we only “cleave” wood when we “split” it with the grain, or, in scientific language, along the line of cleavage, and then we find many pieces with their divided surfaces identical. So that when wood is “broken,” or “chopped,” we obtain pieces of any width or thickness, with no manner of regularity of fracture, but when “cleaved,” we obtain strips which are often perfectly parallel, that is, of equal thickness throughout their whole length, and of such uniformity of surface that it is difficult or even impossible to distinguish one strip from another. Advantage is taken of these lines of cleavage to procure long and extremely thin even strips from trees of the willow variety for such trades as basket-making.

The same effect is seen in house-coal, which may easily be split the way of the grain (on the lines of cleavage), but is much more difficult and requires greater force to break across the grain. Rocks also show distinct lines of cleavage, and are more readily split one way than another, the line of cleavage or stratum of break being at any angle and not necessarily parallel to its bed.

A striking example of this is seen in slate, which may be split in plates, or laminæ, with great facility, though this property is the result of the pressure to which the rock has been for ages subjected, which has caused a change in the molecules, rather than by “cleavage” as the term is strictly understood, and as existing in minerals. Mica is also another example of laminated cleavage, for given care, and a thin, fine knife to divide the plates, this mineral may be “cleaved” to such remarkably thin sheets as to be unable to sustain the most delicate touch without shattering.

These are well-known examples of simple cleavage, in one definite direction, though in many instances there are several forms and directions of cleavage, but even in these there is generally one part or line in and on which cleavage will take place much more readily than on the others, these planes or lines also showing different properties and angular characters, which, no matter how much fractured, always remain the same. It is this “cleavage” which causes a crystal to reproduce itself exactly, as explained in the last chapter, showing its parent form, shape and characteristics with microscopic perfection, but more and more in miniature as its size is reduced.

This may clearly be seen by taking a very small quantity of such a substance as chlorate of potash. If a crystal of this is examined under a magnifying glass till its crystalline form and structure are familiar, and it is then placed in a test-tube and gently heated, cleavage will at once be evident. With a little crackling, the chlorate splits itself into many crystals along its chief lines of cleavage (called the cleavage planes), every one of which crystals showing under the microscope the identical form and characteristics of the larger crystal from which it came.

The cleavage of minerals must, therefore, be considered as a part of their crystalline structure, since this is caused by cleavage, so that both cleavage and crystalline structure should be considered together. Thus we see that given an unchangeable crystal with cleavage planes evident, it is possible easily to reproduce the same form over and over again by splitting, whereas by simply breaking, the form of the crystal would be lost; just as a rhomb of Iceland spar might be sawn or broken across the middle and its form lost, although this would really be more apparent than real, since it would be an alteration in the mass and not in the shape of each individual crystal. And given further cleavage, by time or a sudden breaking down, even the mass, as mass, would eventually become split into smaller but perfect rhombs.

Much skill is, therefore, required in cutting and fashioning a precious stone, otherwise the gem may be ruined at the onset, for it will only divide along its lines of cleavage, and any mistake in deciding upon these, would “break,” not “split” the stone, and destroy the beauty of its crystalline structure. An example of this was specially seen in the great Cullinan diamond, the splitting of which was perhaps the most thrilling moment in the history of precious stones.[A] The value of the enormous crystal was almost beyond computation, but it had a flaw in the centre, and in order to cut out this flaw it was necessary to divide the stone into two pieces. The planes of cleavage were worked out, the diamond was sawn a little, when the operator, acknowledged to be the greatest living expert, inserted a knife in the saw-mark, and with the second blow of a steel rod, the marvellous stone parted precisely as intended, cutting the flaw exactly in two, leaving half of it on the outside of each divided portion.

The slightest miscalculation would have meant enormous loss, if not ruin, to the stone, but the greatest feat the world has ever known in the splitting of a priceless diamond was accomplished successfully by this skilful expert in an Amsterdam workroom in February, 1908. Some idea of the risk involved may be gathered from the fact that this stone, the largest ever discovered, in the rough weighed nearly 3,254 carats, its value being almost anything one cared to state–incalculable.

These cleavage planes help considerably in the bringing of the stone to shape, for in a broad sense, a finished cut stone may be said to be in the form in which its cleavages bring it. Particularly is this seen in the diamond “brilliant,” which plainly evidences the four cleavage planes. These cleavage planes and their number are a simple means of identification of precious stones, though those possessing distinct and ready cleavages are extremely liable to “start” or “split” on these planes by extremes of heat and cold, accidental blows, sudden shocks and the like.

In stones possessing certain crystalline structure, the cleavage planes are the readiest, often the only, means of identification, especially when the stones are chemically coloured to imitate a more valuable stone. In such cases the cleavage of one stone is often of paramount importance in testing the cleavage of another, as is seen in the perfection of the cleavage planes of calcite, which is used in the polariscope.

It sometimes happens, however, that false conditions arise, such as in substances which are of no form or shape, and are in all respects and directions without regular structure and show no crystallisation even in the minutest particles; these are called amorphous. Such a condition sometimes enters wholly or partially into the crystalline structure, and the mineral loses its true form, possessing instead the form of crystals, but without a crystalline structure. It is then called a pseudomorph, which is a term applied to any mineral which, instead of having the form it should possess, shows the form of something which has altered its structure completely, and then disappeared. For instance: very often, in a certain cavity, fluorspar has existed originally, but, through some chemical means, has been slowly changed to quartz, so that, as crystals cannot be changed in shape, we find quartz existing -undeniably quartz- yet possessing the crystals of fluorspar; therefore the quartz becomes a pseudomorph, the condition being an example of what is termed pseudomorphism. The actual cause of this curious chemical change or substitution is not known with certainty, but it is interesting to note the conditions in which such changes do occur.

It is found that in some cases, the matrix of a certain shaped crystal may, after the crystal is dissolved or taken away, become filled by some other and foreign substance, perhaps in liquid form; or a crystalline substance may become coated or “invested” by another foreign substance, which thus takes its shape; or actual chemical change takes place by means of an incoming substance which slowly alters the original substance, so that eventually each is false and both become pseudomorphs. This curious change often takes place with precious stones, as well as with other minerals, and to such an extent that it sometimes becomes difficult to say what the stone ought really to be called.

Pseudomorphs are, however, comparatively easy of isolation and detection, being more or less rounded in their crystalline form, instead of having sharp, well-defined angles and edges; their surfaces also are not good. These stones are of little value, except in the specially curious examples, when they become rare more by reason of their curiosity than by their utility as gems.

Some also show cleavage planes of two or more systems, and others show a crystalline structure comprised of several systems. Thus calcspar is in the 2nd, or hexagonal, whilst aragonite is in the 4th, the rhombic, system, yet both are the same substance, viz.: carbonate of lime.

Such a condition is called dimorphism; those minerals which crystallise in three systems are said to be trimorphous. Those in a number of systems are polymorphous, and of these sulphur may be taken as an example, since it possesses thirty or more modifications of its crystalline structure, though some authorities eliminate nearly all these, and, since it is most frequently in either the 4th (rhombic) or the 5th (monoclinic) systems, consider it as an example of dimorphism, rather than polymorphism.

These varieties of cleavage affect the character, beauty and usefulness of the stone to a remarkable extent, and at the same time form a means of ready and certain identification and classification.

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.

With regard to diamonds, the manufacture of these has not as yet been very successful. As will be seen on reference to the chapter on “the Origin of Precious Stones,” it is generally admitted that these beautiful and valuable minerals are caused by chemically-charged water and occasionally, though not always, high temperature, but invariably beautified and brought to the condition in which they are obtained by the action of weight and pressure, extending unbroken through perhaps ages of time.

In these circumstances, science, though able to give chemical properties and pressure, cannot, of course, maintain these continuously for “ages,” therefore the chemist must manufacture the jewels in such manner that he may soon see the results of his labours, and though real diamonds may be made, and with comparative ease, from boron in the amorphous or pure state along with aluminium, fused in a crucible at a high temperature, these diamonds are but microscopic, nor can a number of them be fused, or in any other way converted into a large single stone, so that imitation stones, to be of any service must be made of a good clear glass.

The glass for this purpose is usually composed of 53.70 per cent. of red lead, 38.48 per cent. of pure quartz in fine powder, preferably water-ground, and 7.82 per cent. of carbonate of potash, the whole coloured when necessary with metallic oxides of a similar nature to the constituents of the natural stones imitated.

But for colourless diamonds, the glass requires no such addition to tint it. From the formula given is made the material known as “strass,” or “paste,” and stones made of it are mostly exhibited under and amongst brilliant artificial lights.

The mere fact that they are sold cheaply is primâ facie proof that the stones are glass, for it is evident that a diamond, the commercial value of which might be £50 or more, cannot be purchased for a few shillings and be genuine. So long as this is understood and the stone is sold for the few shillings, no harm is done; but to offer it as a genuine stone and at the price of a genuine stone, would amount to fraud, and be punishable accordingly. Some of these “paste,” or “white stones,” as they are called in the trade, are cut and polished exactly like a diamond, and with such success as occasionally to deceive all but experts. Such imitations are costly, though, of course, not approaching the value of the real stones; it being no uncommon thing for valuable jewels to be duplicated in paste, whilst the originals are kept in the strong room of a bank or safe-deposit.

In all cases, however, a hard file will abrade the surface of the false stone. We found that quartz is in the seventh degree of hardness, and an ordinary file is but a shade harder than this, so that almost all stones higher than No. 7 are unaffected by a file unless it is used roughly, so as to break a sharp edge. In order to prepare artificial diamonds and other stones for the file and various tests, they are often what is called “converted” into “doublets” or “triplets.”

These are made as follows: the body of the glass is of paste, and on the “table”, and perhaps on the broader facets, there will be placed a very thin slab of the real stone, attached by cement. In the case of the diamond, the body is clear, but in the coloured imitations the paste portion is made somewhat lighter in shade than the real stone would be, the portion below the girdle being coloured chemically, or mounted in a coloured backing. Such a stone will, of course, stand most tests, for the parts usually tested are genuine.

A stone of this nature is called a “doublet,” and it is evident that when it is tested on the underside, it will prove too soft, therefore the “triplet” has been introduced. This is exactly on the lines of the doublet, except that the collet and perhaps the pavilions are covered also, so that the girdle, which is generally encased by the mounting, is the only surface-portion of paste. In other cases the whole of the crown is genuine, whilst often both the upper and lower portions are solid and genuine, the saving being effected by using a paste centre at the girdle, covered by the mounting.

Such a stone as this last mentioned is often difficult to detect without using severe tests and desperate means, e.g.:

by its crystalline structure,
by the cleavage planes,
by the polariscope,
by the dichroscope,
by specific gravity
cutting off the mounting, and examining the girdle;
soaking the stone for a minute or so in a mixture said to have been originally discovered by M. D. Rothschild, and composed of hydrofluoric acid and ammonia; this will not answer for all stones, but is safe to use for the diamond and a few others.

Should the jewel be glass, it will be etched, if not completely destroyed, but if genuine, no change will be apparent;

-soaking the diamond for a few minutes in warm or cold water, in alcohol, in chloroform, or in all these in turn, when, if a doublet, or triplet, it will tumble to pieces where joined together by the cement, which will have been dissolved. It is, however, seldom necessary to test so far, for an examination under the microscope, even with low power, is usually sufficient to detect in the glass the air-bubbles which are almost inseparable from glass-mixtures, though they do not detract from the physical properties of the glass. The higher powers of the same instrument will almost always define the junction and the layer or layers of cement, no matter how delicate a film may have been used. Any one of these tests is sufficient to isolate a false stone.

Some of the softer genuine stones may be fused together with splinters, dust, and cuttings of the same stones, and of this product is formed a larger stone, which, though manufactured, is essentially perfectly real, possessing exactly the same properties as a naturally formed stone.

Many such stones are obtained as large as an ordinary pin’s head, and are much used commercially for cluster-work in rings, brooches, for watch-jewels, scarf-pins, and the like, and are capable of being cut and polished exactly like an original stone. This is a means of using up to great advantage the lapidary’s dust, and though these products are real stones, perhaps a little more enriched in colour chemically, they are much cheaper than a natural stone of the same size and weight.

Some spurious stones have their colour improved by heat, by being tinged on the outside, by being tinted throughout with a fixed colour and placed in a clear setting; others, again, have a setting of a different hue, so that the reflection of this shall give additional colour and fire to the stone.

For instance, glass diamonds are often set with the whole of the portion below the girdle hidden, this part of the stone being silvered like a mirror. Others are set open, being held at the girdle only, the portion covered by the setting being silvered. Other glass imitations, such as the opal, have a tolerably good representation of the “fiery” opal given to them by the admixture, in the glass, of a little oxide of tin, which makes it somewhat opalescent, and in the setting is placed a backing of red, gold, copper, or fiery-coloured tinsel, whilst the glass itself, at the back, is painted very thinly with a paint composed of well washed and dried fish-scales, reduced to an impalpable powder, mixed with a little pure, refined mastic, or other colourless varnish. This gives a good imitation of phosphorescence, as well as a slight pearliness, whilst the tinsel, seen through the paint and the curious milkiness of the glass, gives good “fire.”

A knowledge of the colours natural to precious stones and to jewels generally is of great service in their rough classification for testing, even though some stones are found in a variety of colours.