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Specific gravity.

GELSENKIRCHEN, a town of Germany in the Prussian | from another. Moreover, it is a character very easily determined province of Westphalia, 27 m. W. of Dortmund on the railway and can be applied to cut stones without injury. The relative Duisburg-Hamm. Pop. (1905) 147,037. It has coal mines, iron weightiness of a stone is called its specific gravity, and furnaces, steel and boiler works, and soap, glass and chemical is often abbreviated as S.G. The number given in factories. In 1903 various neighbouring industrial townships the description of a mineral as S.G. shows how many were incorporated with the town. times the stone is heavier than an equal bulk of the standard with which it is compared, the standard being distilled water at 4° C. If, for example, the S.G. of diamond is said to be 3.5 it means that a diamond weighs 3 times as much as a mass of water of the same bulk. The various methods of determining specific gravity are described under DENSITY. The readiest method of testing precious stones, especially when cut, is to use dense liquids. Suppose it be required to determine whether a yellow

GEM (Lat. gemma, a bud,-from the root gen, meaning "to produce," or precious stone; in the latter sense the Greek term is pos), a word applied in a wide sense to certain minerals which, by reason of their brilliancy, hardness and rarity, are valued for personal decoration; it is extended to include pearl. In a restricted sense the term is applied only to precious stones after they have been cut and polished as jewels, whilst in their raw state the minerals are conveniently called "gem-stones." Some-stone be true topaz or false topaz (quartz), it is merely necessary times, again, the term "gem" is used in a yet narrower sense, being restricted to engraved stones, like seals and cameos.

The subject is treated here in two sections: (1) Mineralogy and general properties; (2) Gems in Art, i.e. engraved gems, such as seals and cameos. The artificial products which simulate natural gem-stones in properties and chemical composition are treated in the separate article GEM, ARTIFICIAL.

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1. MINERALOGY AND GENERAL PROPERTIES The gem-stones form a small conventional group of minerals, including principally the diamond, ruby, sapphire, emerald and opal. Other stones of less value-such as topaz, spinel, chrysoberyl, chrysolite, zircon and tourmaline-are sometimes called fancy stones." Many minerals still less prized, yet often used as ornamental stones,-like moonstone, rock-crystal and agate,occasionally pass under the name of " semi-precious stones," but this is rather a vague term and may include the stones of the preceding group. The classification of gem-stones is, indeed, to some extent a matter of fashion.

Descriptions of the several gem-stones will be found under their respective headings, and the present article gives only a brief review of the general characters of the group.

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to drop the stone into a liquid made up to the specific gravity of about 3; and since topaz has S.G. of 3.5 it sinks in this medium, but as quartz has S.G. of only 2-65 it floats. The densest gem. stone is zircon, which may have S.G. as high as 4-7, whilst the lowest is opal with S.G. 2-2. Amber, it is true, is lighter still, being scarcely denser than water, but this substance can hardly be called à gem.

form and

Although the great majority of precious stones occur crystallized, the characteristic form is destroyed in cutting. The crystal-forms of the several stones are noticed under their respective headings, and the subject is discussed Crystalline fully under CRYSTALLOGRAPHY. A few substances cleavage. used as ornamental stones-like opal, turquoise, obsidian and amber-are amorphous or without crystalline form; whilst others, like the various stones of the chalcedonygroup, display no obvious crystal-characters, but are seen under the microscope to possess a crystalline structure. Gem-stones are frequently found in gravels or other detrital deposits, where they occur as rolled crystals or fragments of crystals, and in many cases have been reduced to the form of pebbles. By the disintegration of the rock which formed the original matrix, its constituent minerals were set free, and whilst many of them were worn away by long-continued attrition, the gem-stones survived by virtue of their superior hardness.

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taken advantage of in dressing the stone before cutting it. The cutting of gem-stones is explained under LAPIDARY.

A high degree of hardness is an essential property of a gemstone, for however beautiful and brilliant a mineral may be it is useless to the jeweller if it lack sufficient hardness to Many crystallized gem-stones exhibit cleavage, or a tendency Hardness. withstand the abrasion to which articles of personal to split in definite directions. The lapidary recognizes a “grain decoration are necessarily subjected. Even if not definitely in the stone. When the cleavage is perfect, as in topaz, it may scratched, the polished stone becomes dull by wear. Imitations render the working of the stone difficult, and produce incipient in paste may be extremely brilliant, but being comparatively cracks in the cut gem. Flaws due to the cleavage planes are soft they soon lose lustre when rubbed. In the article MINERA-called "feathers." The octahedral cleavage of the diamond is LOGY it is explained that the varying degrees of hardness are registered on a definite scale. The exceptional hardness of the diamond gives it a supreme position in this scale, and to it the arbitrary value of 10 has been assigned. The corundum gemstones (ruby and sapphire), though greatly inferior in hardness to the diamond, come next, with the value of 9; and it is notable that the sapphire is usually rather harder than ruby. Then follows the topaz, which, with spinel and chrysoberyl, has a hardness of 8; whilst quartz falls a degree lower. Most gemstones are harder than quartz, though precious opal, turquoise, moonstone and sphene are inferior to it in hardness. Those stones which are softer than quartz. have been called by jewellers demi-dures. To test the hardness of a cut stone, one of its sharp edges may be drawn, with firm pressure, across the smooth surface of a piece of quartz; if it leave a scratch its hardness must be above 7. The stone is then applied in like manner to a fragment of topaz, preferably a cleavage-piece, and if it fail to leave a distinct scratch its hardness is between 7 and 8, whereas if the topaz be scratched it is above 8. An expert may obtain a fair idea of hardness by gently passing the stone over a fine steel file, and observing the feel of the stone and the grating sound which it emits. If a stone be scratched by a steel knife its hardness is below 6. The degree of hardness of a precious stone is soon ascertained by the lapidary when cutting it.

Gem-stones differ markedly among themselves in density or specific weight; and although this is a character which does not directly affect their value for ornamental purposes, it furnishes by its constancy an important means of distinguishing one stone

The beauty and consequent value of gems depend mainly on their colour. Some stones, it is true, are valued for entire absence of colour, as diamonds of pure "water." Colour. Certain kinds of sapphire and topaz, too, are water clear," as also is pure rock-crystal; but in most stones colour is a prime element of attraction. The colour, however, is not generally an essential property of the mineral, but is due to the presence of foreign pigmentary matter, often in very small proportion and in some cases eluding determination. Thus, corundum when pure is colourless, but the presence of traces of certain mineral substances imparts to it not only the red of ruby and the blue of sapphire, but almost every other colour. The tinctorial matter may be distributed either uniformly throughout the stone or in regular zones, or in quite irregular patches. A tourmaline, for instance, may be red at one end of a prismatic crystal and green at the other extremity, or the colour may be so disposed that in transverse section the centre will be red and the outer zone green. A beryl may be yellow and green in the same crystal. Sapphire, again, is often parti-coloured, one portion of the stone being blue and other portions white or yellow; and the skilful lapidary, in cutting the stone, will take advantage of the blue portion. The character of the pigment is in many cases not definitely known. It by no means follows that the material capable of imparting a certain tint to glass is identical with that which naturally colours a stone of the same tint; thus a glass of sapphire-blue may be obtained by the use of cobalt, yet cobalt

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36. Lyre Player. (Cockerell Coll.)

55-57.-GREEK GEMS.

55. Girl with Scroll and Lyre. 57. Head of Aristippus-Deities. 5S-61.-SIGNED GEMS.

58. Asclepius of Aulos.
60. Medusa of Solon.

35-44-FINEST GREEK SCARABS AND SCARABAEOIDS. 62-70.-ROMAN GEMS.

35. Head of Young Warrior.

37. Crane, with Deer's Antler.

38. Head of Eos.

39. Lyre Player. (Woodhouse Coll. and B.M.)
40. Lyre Player, signed by Syries.

41. Stork and Grasshopper, signed by Dexamenos. (St.
Petersburg.)

42. Flying Crane, signed by Dexamenos. (St. Petersburg.) 43. Flying Goose. 44. Lion and Stag.

45-54.-ETRUSCAN SCARABS.

45. Achilles in Retirement.
47. Capaneus struck by the Bolt.
49. Capaneus struck by the Bolt.
51. Heracles and Cycnus.

53. Heracles and the Lion.
54. Machaon bandaging Philoctetes.

46. Victory. 48. Heracles. 50. Achilles. 52. Heracles.

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56. Girl with Water-Jar.

59. Citharist of Allion.

61. Heracles of Gnaios.

63. Head of Trajan Decius.

65. Jupiter of Heliopolis. 67. So-called Psyche. Stamp for the Eye Balsam of

72. Good Shepherd. Jonah.

73-76.-EIGHTEENTH CENTURY GEMS.

73. Achilles of Pamphilus, copied from the antique. 74. Eros and Psyche, by Pichler.

75. Head of Athena.

76. Athena, from Townley Bust by Marchant.

All the above are in the British Museum, unless otherwise stated.

Dichro

ism.

A very useful means of discriminating between certain stones is found in their dichroism, or, to use a more general term, pleochroism. Neither amorphous minerals, like opal, nor minerals crystallizing in the cubic system, like spinel and garnet, possess this property; but coloured minerals which are doubly refracting may show different colours, when properly examined, in different directions. Occasionally this is so marked as to be detected by the naked eye, as in iolite or dichroite, but usually the stone needs to be examined with such an instrument as Haidinger's dichroscope (see CRYSTALLOGRAPHY). It must be remembered that in the direction of an optic axis the two images will be of the same colour in all positions of the instrument, and it is therefore necessary before reaching a definite conclusion to turn the stone about and examine it in various directions. The use of the dichroscope is so simple that it can be applied by any one to the examination of a cut stone, but there are other means of determining the nature of a stone by its optical properties available to the mineralogist and more suitably discussed under CRYSTALLOGRAPHY.

has not been detected in the sapphire. Probably the most common | absorption. It is sometimes useful to examine the behaviour mineral pigments are compounds of iron, manganese, copper and of a stone under the action of the Röntgen rays. chromium. If the colour of the stone be discharged by heat, an organic pigment is presumably present. Some ornamental stones change their colour, or even lose it, on exposure to sunlight and air: such is the case with rose-quartz, chrysoprase and certain kinds of topaz and turquoise. Exposure to heat alters the colour of some stones so readily that the change is taken advantage of commercially; thus, sherry-yellow topaz may be rendered pink, smoky and amethystine quartz may become yellow, and coloured zircons may be decolorized, so as to resemble diamonds. The colours of some gem-stones are greatly affected by radioactivity, and Prof. F. Bordas has found this to be particularly the case with sapphire. From his experiments he believes that yellow corundum, or oriental topaz, may have been formed from blue corundum under the influence of radioactive substances present in the soil in which the sapphire was embedded. Different shades of colour may be presented by different stones of the same species; and it was formerly the custom of lapidaries to regard the darker stones as masculine and the paler as feminine, a full blue sapphire, for instance, being called a "male sapphire and a delicate blue stone a "female sapphire." It is notable that some stones appear to change colour by candle-light and by most other artificial means of illumination; some amethysts thus become inky, and certain sapphires acquire a murky tint, whilst others become amethystine. For an example of a remarkable change of this character, see ALEXANDRITE.

Refrac tion.

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Chemical composi tion.

In chemical composition the gem-stones present great variety. Diamond is composed of only a single element; ruby, sapphire and the quartz-group are oxides; spinel and chrysoberyl may be regarded as aluminates; turquoise and beryllonite are phosphates; and a great number of ornamental stones are silicates of greater or less complexity, such as emerald, topaz, chrysolite, garnet, zircon, tourmaline, kunzite, sphene and benitoite. In the examination of a cut stone chemical tests are not available, since they usually involve the partial destruction of the mineral. The artificial production of certain gems by chemical processes which yield products identical in composition and physical properties with the natural stones, is described in the article GEM, ARTIFICIAL.

Doublets and triplets are composite stone, sometimes prepared for fraudulent purposes. In a doublet a slab of real gem-stone covers the face of a paste, whilst in a triplet the paste is both

a suitable solvent, such as chloroform or in some cases even hot water, the cement uniting the pieces gives way and the compound character of the structure is detected.

As the optical properties of minerals are fully explained under CRYSTALLOGRAPHY, little need be said here on this subject. The brilliancy of a cut stone depends on the amount of light reflected from its faces; and in the form known as the "brilliant" the gem is so cut that much of the incident light, after entering the stone and suffering refraction, is totally reflected from the facets at the back. The amount of light which is thus returned to the eye of the observer will be greater as the angle of total reflection, or critical angle, is smaller, but this angle will be small if the refractive power of the stone is great, so that the brilliancy directly depends on the re-faced and backed by a slice of genuine stone. By the action of fractivity. The diamond has the highest refractive index of any gem-stone (2.42). Jargoon, or zircon, has also a high index (mean 1-95), and sphene, which is occasionally cut as a gem, is likewise very notable in this respect. The index of refraction generally bears a relation to the specific gravity of the stone, the heaviest gems having the highest indices, though a few minerals offer exceptions. The refractive index, which is thus a very important character in the scientific discrimination of gem-stones, may be conveniently determined, within certain limits, by means of the refractometer devised by Dr G. F. Herbert Smith. This instrument is an improved form of the total reflectometer, in which the refractive power of a given substance is determined by the method of total reflection. It may be used for indices ranging from 1.300 to 1.775, and may be applied to faceted stones without removal from their settings. The play of prismatic colours exhibited by a cut stone, often known as its "fire," is due to the decomposition of the white light which enters the stone, and is returned, by internal Disperreflection, after resolution into its coloured components. sion. This decomposition depends on the dispersive power of the substance. The exceptional beauty of the fiery flashes in the diamond is due to its high dispersion, in other words, to the difference between the refractive indices for the red rays and the violet rays at the extremities of the spectrum. The peculiar lustre exhibited by the diamond is called adamantine, and is shared to some extent by certain other stones which have a high refractive index and high dispersion, such as zircon.

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Before the chemical composition of gem-stones was understood, their classification remained vague and unscientific. As the ancients depended almost entirely on the eye, the colour of the stone naturally became the chief factor in classification. A variety of stones agreeing roughly in colour would be grouped together under a common name, widely as they might differ in other respects. Thus the emerald, the peridot, green fluorspar, malachite, and certain kinds of quartz and jade seem to have been united under the general name of oμápaydos; whilst the ruby, red spinel and garnet were probably grouped together as carbunculus. In this way minerals radically different were associated on the ground of what is generally a superficial and accidental character, and rarely of any classificatory value. On the other hand, a grouping based only on colour led to several names being in some cases applied to the same mineral species. Thus the ruby and sapphire are essentially identical in chemical composi tion and in all physical characters, save colour.

Supersti. tions.

Descriptions of precious stones by ancient writers generally are too vague for exact diagnosis. The principal classical authorities are Theophrastus and the elder Pliny. Stones were formerly held in esteem not only for their beauty and rarity but for the medicinal and magical powers with which they were reputed to be endowed. Up to comparatively recent years the toadstone, for example, was worn not for beauty but for sake of occult virtue; and even at the present day certain stones, like jade, are valued for a similar reason. Prof. W. Ridgeway has suggested that jewelry took its origin not, as often supposed, in an innate love of personal decoration, but rather in the belief that the objects used possessed magical virtue. Small stones peculiar in colour or shape, especially those with natural perforations, are usually valued by uncivilized peoples

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as amulets. The Orphic poem Atiká, reputed to be of very early fortunately the relief is incomplete, and the published illustrathough unknown date, is rich in allusions to the virtues of many tion inadequate. It would seem, however, that a revolving tool of the gem-stones. Many of the medical and other virtues of was supported by a kind of mandrel, and actuated in primitive precious stones were evidently attributed to them on the well- fashion by a bow. An alternative plan of working was to use a known doctrine of signatures. Thus, the blood-red colour of a splinter of diamond set in a handle and applied like a graver. fine jasper suggested that the stone would be useful in haemor- Both systems are clearly indicated by Pliny, who in one passage rhage; a green jasper would bring fertility to the soil; and the (H.N. xxxvii. 60) states that diamond splinters are sought out by purple wine-colour of amethyst pointed to its value as a pre-gem engravers and set in iron, and so easily hollow out stones of ventive of intoxication. Many of the superstitions came down to modern times, and even at the present day the belief in "lucky stones" is by no means extinct.

BIBLIOGRAPHY.-The most comprehensive work on gem-stones is Professor Max Bauer's Edelsteinkunde (1896), translated, with additions, by L. J. Spencer under the title Precious Stones (1904). Less detailed are Professor P. Groth's Grundriss der Edelsteinkunde (1887) and Professor C. Doelter's Edelsteinkunde (1893). Sir A. H. Church's Precious Stones (1905), intended as a guide to the collections in the Victoria and Albert Museum, is a convenient introduction; and Professor H. A. Miers's Cantor Lectures at the Society of Arts on Precious Stones (1896) may be studied with advantage. For American stones, the valuable work of Dr G. F. Kunz, The Gems and Precious Stones of N. America, is a standard authority; and the Annual Reports of this writer and others, published by the Geological Survey of the United States in the Mineral Resources, form a repertory of valuable information on precious stones in general. The articles in The Mineral Industry (founded by R. P. Rothwell) should also be consulted. See likewise O. C. Farrington, Gems and Gem Minerals (Chicago, 1903). For optical characters reference should be made to G. F. H. Smith, The Herbert Smith Refractometer (London, 1907); L. Claremont, The GemCutter's Craft (London, 1906); W. Goodchild, Precious Stones (London, 1908). (F. W. R.")

2. GEMS IN ART

In art, the word Gem is the general term for precious stones when engraved with designs, whether adapted for sealing (oopayis, sigillum, intaglio), or mainly for artistic effect (imagines eclypae, cameo). They exist in a very large number of undoubtedly genuine old examples, extending from the mists of Babylonian antiquity to the decline of Roman civilization, and again starting with a new, but less original impulse on the revival of art. Apart from workmanship they possess the charms of colour deep, rich, and varied, of material unequalled for its endurance, and of scarcity, which in many instances has been enhanced by the remoteness of the lands whence they came or the fortuity of their occurrence. These qualities united within the small compass of a gem were precisely such as were required in a seal as a thing of constant use, so inalienable in its possession as to become naturally a personal ornament and an attractive medium of artistic skill, no less than the centre of traditions or of religious and legendary associations. As regards the nations of classical antiquity, all seals are classed as gems, though in many cases the material is not such as would strictly come under that heading, and precious stones in the modern sense are hardly known to occur. On the other hand it must not be supposed that gems engraved in intaglio were necessarily employed as seals. At all periods many intaglios are found which could not have been so employed without great difficulty. In Greece and Rome, within historic times, gems were worn engraved with designs to show that the bearer was an adherent of a particular worship, the follower of a certain philosopher, or the attached subject of an emperor. However, speaking generally, the intaglio engraving is a means to an end, namely, a seal-impression, while an engraving in relief is complete in itself.

Methods of Engraving (see also under LAPIDARY).-In gem engraving the principal modern implement is a wheel or minute copper disk, driven in the manner of a lathe, and moistened with olive oil mixed with emery or diamond dust. There is no clear proof of the use among the ancients of a wheel mounted lathe wise, but we have abundant indications of drilling with a revolving tool, which might be either a tubular drill making a ring-like depression, a pointed tool making a cup-like sinking, or a small wheel with a cutting edge, making a boat-shaped depression.

We have one sepulchral monument from Philadelphia showing the tool of an intaglio engraver (dakтuλokoiλoyúpos; see Athenische Mitteilungen des Arch. Inst. xv. p. 333). Un

any degree of hardness; while elsewhere (H.N. xxxvii. 200) he speaks of the special efficacy of the fervor terebrarum, the vehement action of drills. A third method is also indicated by Pliny (ibid.) when he speaks of the use of a blunted tool, which must have been moistened and supplied with emery of Naxos.

A four-sided pendant of the Hellenistic period published by Furtwängler (Antike Gemmen, Gesch. p. 400) shows clearly the successive stages of the operation. On side a the subject is slightly sketched in with the diamond point. On side b the deepest parts of the figure have also been roughly scooped out with the wheel. On sides c and d the wheel work is fairly complete, but the finer internal work has not been begun.

After the design had been completed the stone must have received a final polish on its surface, to obliterate any erroneous strokes of the first sketch; but this process was not carried as far as in modern work. It is a popular error to suppose that a high degree of internal polish is a proof of antiquity. If the interior of the design has a high degree of polish it may be either ancient or modern, or it may be an ancient stone repolished in modern times. If it has a matt surface uniformly produced by intention, it is probably modern. If the design is slightly dimmed and worn or scratched the stone may be antique, but is not necessarily so, since modern engravers have observed this peculiarity, and have imitated it with a success which, were there no other grounds of suspicion, might escape detection.

History. It has been a subject of controversy whether the first infancy of the art was passed in Egypt or in Babylonia, but it seems highly probable that it was developed in Babylonia, whence at any rate the oldest examples of engraved gems at present known are obtained. It does not necessarily follow, however, that Egypt was therefore a pupil. It may well be that the art was developed independently in the two countries, although certain points of possible contact in respect of the forms employed will be described below in the section dealing with primitive Egypt.

Babylonia.-At a very remote period the cylindrical form of stone was introduced and became the approved shape, while the technical skill of the artist was still slight, and the traces of the tools employed (drill and pencil point) were still unconcealed.

The cylinder was suspended by a string and used as a seal. Impressions of cylinders are frequent on contract tablets. If one of the parties cannot use a seal he makes a nail-mark in lieu thereof, as is recorded in the document.

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But from a time that was still comparatively early the en gravers could work with considerable skill in the hard stone. particular a cylinder may be quoted in the de Clercq Collection bearing the name of Sargon I. of Agade, who is placed about 3500 B.C. The cylinder is engraved with the king's name and titles and two symmetrically disposed renderings of Izdubar, with a vase of flowing water giving drink to a bull. The whole is treated in a conventionalized style that indicates long traditions. An important early cylinder in the British Muscum is inscribed with the name of a viceroy of Ur-Gur, king of Ur (about 2500 B.C.). The engraving shows Ur-Gur being led into the presence of Sin, the moon-god.

The cylinder seal was adopted by the Assyrians, and so was carried on continuously till the time of the Persian conquest of Babylon (538 B.C.). Meanwhile, as an alternative form the conoidal seal, rounded at the top and having a flat base for the intaglio, came into use beside the cylinder.

In style the Assyrians carried on the Babylonian tradition, but with no freedom of design. Subjects and treatment became rigidly conventional.

After the Persian conquest the victors adopted the cylinder

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