Part 4 (1/2)
For the purpose under consideration, the temperature of the pencil of heat applied to the stones in their several portions was kept constant.
It will be seen, therefore, that no great reliance can be placed on the heat test as applied to precious stones.
CHAPTER X.
PHYSICAL PROPERTIES.
H--MAGNETIC AND ELECTRIC INFLUENCES.
The word ”electricity” is derived from the Greek ”elektron,” which was the name for amber, a mineralised resin of extinct pine-trees. It was well-known to the people of pre-historic times; later to the early Egyptians, and, at a still later date, we have recorded how Thales--the Greek philosopher, who lived about the close of the 7th Century B.C., and was one of the ”seven wise men”--discovered the peculiar property which we call ”electricity” by rubbing dry silk on amber.
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 versa; others again, when heated, become powerfully magnetic and a.s.sume 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 a.s.sumes 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 gla.s.s 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 ill.u.s.trated 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 magnet.i.te, 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, FeS_{2}, is unresponsive, whilst the magnetic pyrites, which varies from 5FeS, Fe_{2}S_{3}, to 6FeS, Fe_{2}S_{3}, and is a sulphide of iron, is responsive both positively and negatively. Bis.m.u.th 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 quant.i.ty 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 quant.i.ty of iron to bring it the same a.n.a.lytically 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.
CHAPTER XI.
THE CUTTING OF PRECIOUS STONES.
As existing in a state of nature precious stones do not, as a rule, exhibit any of those beautiful and wonderful properties which cause them to be so admired and sought after as to become of great intrinsic value, for their surfaces have become clouded by innumerable fine cuts or abrasions, because of the thousands of years during which they have been under pressure, or tumbled about in rivers, or subjected to the incessant friction caused by surrounding substances. All this occurring above and under ground has given them an appearance altogether different to that which follows cutting and polis.h.i.+ng. Further, the shape of the stone becomes altered by the same means, and just as Michael Angelo's figure was already in the marble, as he facetiously said, and all he had to do was to chip off what he did not require till he came to it, so is the same process of cutting and polis.h.i.+ng necessary to give to the precious stones their full value, and it is the manner in which these delicate and difficult operations are performed that is now under consideration. Just as experience and skill are essential to the obtaining of a perfect figure from the block of marble, so must the cutting and polis.h.i.+ng of a precious stone call for the greatest dexterity of which a workman is capable, experience and skill so great as to be found only in the expert, for in stones of great value even a slight mistake in the shaping and cutting would probably not only be wasteful of the precious material, but would utterly spoil its beauty, causing incalculable loss, and destroying altogether the refrangibility, l.u.s.tre and colour of the stone, thus rendering it liable to easy fracture: in every sense converting what would have been a rare and magnificent jewel to a comparatively valueless specimen.
One of the chief services rendered by precious stones is that they may be employed as objects of adornment, therefore, the stone must be cut of such a shape as will allow of its being set without falling out of its fastening--not too shallow or thin, to make it unserviceable and liable to fracture, and in the case of a transparent stone, not too deep for the light to penetrate, or much colour and beauty will be lost. Again, very few stones are flawless, and the position in which the flaw or flaws appear will, to a great extent, regulate the shape of the stones, for there are some positions in which a slight flaw would be of small detriment, because they would take little or no reflection, whilst in others, where the reflections go back and forth from facet to facet throughout the stone, a flaw would be magnified times without number, and the value of the stone greatly reduced. It is therefore essential that a flaw should be removed whenever possible, but, when this is not practicable, the expert will cut the stone into such a shape as will bring the defect into the least important part of the finished gem, or probably sacrifice the size and weight of the original stone by cutting it in two or more pieces of such a shape that the cutting and polis.h.i.+ng will obliterate the defective portions. Such a method was adopted with the great Cullinan diamond, as described in Chapter IV. From this remarkable diamond a great number of magnificent stones were obtained, the two chief being the largest and heaviest at present known. Some idea of the size of the original stone may be gathered from the fact that the traditional Indian diamond, the ”Great Mogul,” is said to have weighed 280 carats. This stone, however, is lost, and some experts believe that it was divided, part of it forming the present famous Koh-i-nur; at any rate, all trace of the Great Mogul ceased with the looting of Delhi in 1739. The Koh-i-nur weighs a little over 106 carats; before cutting it weighed a shade over 186; the Cullinan, in the same state, weighed nearly 3254 carats. This ma.s.sive diamond was cut into about 200 stones, the largest, now placed in ”The Royal Sceptre with the Cross,” weighing 516-1/2 carats, the second, now placed under the historic ruby in ”The Imperial State Crown,” weighing 309-3/16ths carats. These two diamonds are now called ”The Stars of Africa.” Both these stones, but especially the larger, completely overshadow the notorious Koh-i-nur, and notwithstanding the flaw which appeared in the original stone, every one of the resulting pieces, irrespective of weight, is without the slightest blemish and of the finest colour ever known, for the great South African diamond is of a quality never even approached by any existing stone, being ideally perfect.
It requires a somewhat elaborate explanation to make clear the various styles of cut without ill.u.s.trations. They are usually divided into two groups, with curved, and with flat or plane surfaces. Of the first, the curved surfaces, opaque and translucent stones, such as the moonstone, cat's-eye, etc., are mostly cut _en cabochon_, that is, dome-shaped or semi-circular at the top, flat on the underside, and when the garnet is so cut it is called a carbuncle. In strongly coloured stones, while the upper surface is semi-circular like the cabochon, the under surface is more or less deeply concave, sometimes following the curve of the upper surface, the thickness of the stone being in that case almost parallel throughout. This is called the ”hollow” cabochon. Other stones are cut so that the upper surface is dome-shaped like the last two, but the lower is more or less convex, though not so deep as to make the stone spherical. This is called the ”double” cabochon.
A further variety of cutting is known as the _goutte de suif_, or the ”tallow-drop,” which takes the form of a somewhat flattened or long-focus double-convex lens. The more complicated varieties of cut are those appearing in the second group, or those with plane surfaces. A very old form is the ”rose” or ”rosette”; in this the extreme upper centre, called the ”crown,” or ”star,” is usually composed of six triangles, the apexes of which are elevated and joined together, forming one point in the centre. From their bases descend a further series of triangles, the bases and apexes of which are formed by the bases and lower angles of the upper series. This lower belt is called the ”teeth,”
under which the surface or base of the stone is usually flat, but sometimes partakes of a similar shape to the upper surface, though somewhat modified in form.