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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
gravity. 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 GEM (Lat. gemma, a bud,--from the root gen, meaning with which it is compared, the standard being distilled water at " to produce," -or precious stone; in the latter sense the Greek 4° C. If, for example, the S.G. of diamond is said to be 3.5 it term is vidos), a word applied in a wide sense to certain minerals means that a diamond weighs 3 times as much as a mass of water which, by reason of their brilliancy, hardness and rarity, are valued of the same bulk. The various methods of determining specific for personal decoration; it is extended to include pearl. In a gravity are described under Density. The readiest method of restricted sense the term is applied only to precious stones after testing precious stones, especially when cut, is to use dense they have been cut and polished as jewels, whilst in their raw liquids. Suppose it be required to determine whether a yellow 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, to drop the stone into a liquid made up to the specific gravity of being restricted to engraved stones, like seals and cameos. about 3; and since topaz has S.G. of 3.5 it sinks in this mcdium,
The subject is treated here in two sections: (1) Mineralogy but as quartz has S.G. of only 2.65 it floats. The densest gem. and general properties; (2) Gems in Art, i.e. engraved gems, such stone is zircon, which may have S.G. as high as 4•7, whilst the as seals and cameos. The artificial products which simulate lowest is opal with S.G. 2.2. Amber, it is true, is lighter still, natural gem-stones in properties and chemical composition are being scarcely denser than water, but this substance can hardly treated in the separate article GEM, ARTIFICIAL.
be called à gem.
Although the great majority of precious stones occur crystal1. MINERALOGY AND GENERAL PROPERTIES
lized, the characteristic form is destroyed in cutting. The The gem-stones form a small conventional group of minerals, crystal-forms of the several stones are noticed under including principally the diamond, ruby, sapphire, emerald and their respective headings, and the subject is discussed Crystallilae
form and opal. Other stones of less value---such as topaz, spinel, chryso- fully under CRYSTALLOGRAPHY. A few substances cleavage. beryl, chrysolite, zircon and tourmaline-are sometimes called used as ornamental stones-like opal, turquoise, "fancy stones.” Many minerals still less prized, yet often used obsidian and amber-are amorphous or without crystalline as ornamental stones,-like moonstone, rock-crystal and agate,- form; whilst others, like the various stones of the chalcedonyoccasionally pass under the name of " semi-precious stones," group, display no obvious crystal-characters, but are seen under but this is rather a vague term and may include the stones of the the microscope to possess a crystalline structure. Gem-stones preceding group. The classification of gem-stones is, indeed, to are frequently found in gravels or other detrital deposits, where some extent a matter of fashion.
they occur as rolled crystals or fragments of crystals, and in Descriptions of the several gem-stones will be found under many cases have been reduced to the form of pebbles. By the their respective headings, and the present article gives only a disintegration of the rock which formed the original matrix, its brief review of the general characters of the group.
constituent minerals were set free, and whilst many of them A high degree of hardness is an essential property of a gem were worn away by long-continued attrition, the gem-stones stone, for however beautiful and brilliant a mineral may be it is survived by virtue of their superior hardness.
useless to the jeweller if it lack sufficient hardness to Many crystallized gem-stones exhibit cleavage, or a tendency
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 taken advantage of in dressing the stone before cutting it. The registered on a definite scale. The exceptional hardness of the cutting of gem-stones is explained under LAPIDARY. diamond gives it a supreme position in this scale, and to it the The beauty and consequent value of gems depend mainly arbitrary value of 10 has been assigned. The corundum gem on their colour. Some stones, it is true, arc valued for entire stones (ruby and sapphire), though greatly inferior in hardness absence of colour, as diamonds of pure “ water."
Colour. to the diamond, come next, with the value of 9; and it is notable Certain kinds of sapphire and topaz, too, are "water that the sapphire is usually rather harder than ruby. Then clear,” as also is pure rock-crystal; but in most stones colour is a follows the topaz, which, with spinel and chrysoberyl, has a prime element of attraction. The colour, however, is not generally hardness of 8; whilst quartz falls a degree lower. Most gem an essential property of the mineral, but is due to the presence of stones are harder than quartz, though precious opal, turquoise, foreign pigmentary matter, often in very small proportion and in moonstone and sphene are inscrior to it in hardness. Those some cases eluding determination. Thus, corundum when pure stones which are softer than quartz, have been called by jewellers is colourless, but the presence of traces of certain mineral subdemi-dures. To test the hardness of a cut stone, one of its sharp stances imparts to it not only the red of ruby and the blue of edges may be drawn, with firm pressure, across the smooth sapphire, but almost every other colour. The tinctorial matter surface of a piece of quartz; if it leave a scratch its hardness must may be distributed either uniformly throughout the stone or in be above 7. The stone is then applied in like manner to a regular zones, or in quite irregular patches. A lourmaline, for fragment of topaz, preferably a cleavage-piece, and if it fail to instance, may be red at one end of a prismatic crystal and green leave a distinct scratch its hardness is between 7 and 8, whereas at the other extremity, or the colour may be so disposed that in is the topaz be scratched it is above 8. An expert may obtain a transverse section the centre will be red and the outer zone fair idea of hardness by gently passing the stone over a fine green. A beryl may be yellow and green in the same crystal. steel file, and observing the feel of the stone and the grating Sapphire, again, is often parti-coloured, one portion of the stone sound which it emits. If a stone be scratched by a steel knife its being blue and other portions white or yellow; and the skilful hardness is below 6. The degree of hardness of a precious stone lapidary, in cutting the stone, will take advantage of the blue is soon ascertained by the lapidary when cutting it.
portion. The character of the pigment is in many cases not Gem-stones differ markedly among themselves in density or definitely known. It by no means follows that the material specific weight; and although this is a character which does not capable of imparting a certain tint to glass is identical with that directly affect their value for ornamental purposes, it furnishes which naturally colours a stone of the same tint; thus a glass of by its constancy an important means of distinguishing one stone I sapphire-blue may be obtained by the use of cobalt, yet cobale
16-18.-GEMS OF THE ISLANDS, Babylonian (late Sumerian) Cylinder of a Viceroy
16. Goddess on Waves. Birds. of Ur-Gur (or Ur-Engur), 2500 B.C.
17. Lion and Goat. 2. Assyrian Cylinder. Woman adoring Goddess.
13. Heracles and Nereus. 3. Assyrian Cylinder. Assur worshipped by two
Assyrian kings, and divine Attendants. 4. Persian Seal of Darius (500 B.C.). Lion Hunt.
19.-PHOENICIAN SEAL, inscribed. 5. Graeco-Persian Scarabaeoid. Boar Hunt.
20-26.-GRAECO-PHOENICIAN SCARABS 6-15.-CRETAN AND MYCENAEAN INTAGLIOS. 6. Cretan Symbols.
THARROS. 7. Man and Bull. Crete.
20. King, enthroned.
21. 8. Lions and Column. Ialysus.
Bes with Antelope and Hound.
22. Bes with Lions. 9. Daemon. Crete.
23. Warrior. Lioness and Deer. II-13. Three-sided Stone. Peloponnesus.
24. Egyptian Device.
25. Bes and Goats. 14. Man and Bull. Crete.
26. Hawk of Horus. 15. Bull and Palm. Ialysus.
All the above are in the British Museum.
27-34.-EARLY GREEK SCARABS AND SCARABAEOIDS. 55-57.-GREEK GEMS.
27. Pluto and Persephone. (New York.) 28. Boreas and Oreithyia. (New York.)
55. Girl with Scroll and Lyre. 56. Girl with Water-Jar.
57. Head of Aristippus-Deities. 29. Youth and Dog. 30. Archer feeling Arrow Tip. (Lord Southesk.)
53–61.-SIGNED GEMS. 31. Satyr and Wine Cup. 32. Archer and Dog.
58. Asclepius of Aulos. 59. Citharist of Allion. 33. Satyr with Wineskin.
60. Medusa of Solon. 34. Athena with Gorgon Spoils.
61. Heracles of Gnaios. 35-44.-FINEST GREEK SCARABS AND SCARABAEOIDS. 62–70.-ROMAN GEMS. 35. Head of Young Warrior.
62. Portrait 36. Lyre Player. (Cockerell Coll.)
63. Head of Trajan Decius.
64. Ares and Aphrodite. 37. Crane, with Deer's Antler.
65. Jupiter of Heliopolis. 38. Head of Eos.
66. Artemis of Ephesus. 39. Lyre Player. (Woodhouse Coll. and B.M.)
67. So-called Psyche. 40. Lyre Player, signed by Syries.
63. So-called Psyche. 41. Stork and Grasshopper, signed by Dexamenos. (St.
6.). Minerva with Mask, Stamp for the Eye Balsam of Petersburg)
Herophilus. 42. Flying Crane, signed by Dexamenos. (St. Petersburg.)
70. Helios. 43. Flying Goose. 44. Lion and Stag.
71-72.-CHRISTIAN GEMS. 45-54.-ETRUSCAN SCARABS.
71. Crucifixion. 72. Good Shepherd. Jonah. 45. Achilles in Retirement.
46. Victory. 47. Capaneus struck by the Bolt, 48. Heracles. 73-76.--EIGHTEENTH CENTURY GEMS. 49. Capaneus struck by the Bolt.
73. Achilles of Pamphilus, copied from the antique. 51. Heracles and Cycnus.
74. Eros and Psyche, by Pichler. 53. Heracles anii the Lion.
75. Head of Athena. 54. Machaon bandaging Philoctetes.
56. Athena, from Townley Bust by Marchant. All the above are in the British Museum, unless otherwise stated.
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 A very useful means of discriminating between certain stones organic pigment is presumably present. Some ornamental stones is found in their dichroism, or, to use a more general term, change their colour, or even lose it, on exposure to sunlight and pleochroism. Neither amorphous minerals, like opal, air: such is the case with rose-quartz, chrysoprase and certain nor minerals crystallizing in the cubic system, like kinds of topaz and turquoise. Exposure to heat alters the colour spinel and garnet, possess this property; but coloured of some stones so readily that the change is taken advantage minerals which are doubly refracting may show different colours, of commercially; thus, sherry-yellow topaz may be rendered when properly examined, in different directions. Occasionally pink, smoky and amethystine quartz may become yellow, and this is so marked as to be detected by the naked eye, as in iolite coloured zircons may be decolorized, so as to resemble diamonds. or dichroite, but usually the stone needs to be examined with such
The colours of some gem-stones are greatly affected by radio- an instrument as Haidinger's dichroscope (see CRYSTALLOactivity, and Prof. F. Bordas has found this to be particularly CRAPHY). It must be remembered that in the direction of an the case with sapphire. From his experiments he believes that optic axis the two images will be of the same colour in all positions yellow corundum, or oriental topaz, may have been formed from of the instrument, and it is therefore necessary before reaching blue corundum under the influence of radioactive substances a definite conclusion to turn the stone about and examine present in the soil in which the sapphire was embedded. Different it in various directions. The use of the dichroscope is so shades of colour may be presented by different stones of the same simple that it can be applied by any one to the examination species; and it was formerly the custom of lapidațies to regard of a cut stone, but there are other means of determining the nature the darker stones as masculine and the paler as feminine, a full of a stone by its optical properties available to the mineralogist blue sapphire, for instance, being called a “male sapphire ” and more suitably discussed under CRYSTALLOGRAPHY. and a delicate blue stone a “ female sapphire." It is notable In chemical composition the gem-stones present grcat variety. that some stones appear to change colour by candle-light and Diamond is composed of only a single element; ruby, sapphire by most other artificial means of illumination; some amethysts and the quartz-group are oxides; spinel and chrysothus become inky, and certain sapphires acquire a murky tint, beryl may be regarded as aluminates; turquoise and
compostwhilst others become amethystine. For an example of a remark- beryllonite are phosphates; and a great number of able change of this character, see ALEXANDRITE.
ornamental stones are silicates of greater or less As the optical properties of minerals are fully explained under complexity, such as emerald, topaz, chrysolite, garnet, zircon, CRYSTALLOGRAPHY, little need be said here on this subject. tourmaline, kunzite, sphene and benitoite. In the cxamination
The brilliancy of a cut stone depends on the amount of a cut stone chemical tests are not available, since they usually Refrac
of light reflected from its faces; and in the form involve the partial destruction of the mineral. The artificial
known as the “brilliant” the gem is so cut that much production of certain gems by chemical processes which yield of the incident light, after entering the stone and suffering products identical in composition and physical properties with refraction, is totally reflected from the facets at the back. The the natural stones, is described in the article GEM, ARTIFICIAL. amount of light which is thus returned to the eye of the observer Doublets and triplets are composite stone, sometimes prepared will be greater as the angle of total reflection, or critical angle, is for fraudulent purposes. In a doublet a slab of real gem-stone smaller, but this angle will be small if the refractive power of the covers the face of a paste, whilst in a triplet the paste is both stone is grcat, 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 a suitable solvent, such as chloroform or in some cases even hot gem-stone (2:42). Jargoon, or zircon, has also a high index water, the cement uniting the pieces gives way and the compound (mean 1.95), and sphene, which is occasionally cut as a gem, is character of the structure is detected. likewise very notable in this respect. The index of refraction Before the chemical composition of gem-stones was understood, generally bears a relation to the specific gravity of the stone, their classification remained vague and unscientific. As the the heaviest gems having the highest indices, though a few ancients depended almost entirely on the eye, the colour of the minerals offer exceptions. The refractive index, which is thus stone naturally became the chief factor in classification. A a very important character in the scientific discrimination of variety of stones agreeing roughly in colour would be grouped gem-stones, may be conveniently determined, within certain together under a common name, widely as they might differ in limits, by means of the refractometer devised by Dr G. F. other respects. Thus the emerald, the peridot, green fluorspar, Herbert Smith. This instrument is an improved form of the malachite, and certain kinds of quartz and jade seem to have been total reflectometer, in which the refractive power of a given united under the general name of quápaydos; whilst the ruby, substance is determined by the method of total reflection. It red spinel and garnet were probably grouped together as car. may be used for indices ranging from 1•300 to 1•775, and may bunculus. In this way minerals radically different were associated be applied to faceted stones without removal from their settings. on the ground of what is generally a superficial and accidental
The play of prismatic colours exhibited by a cut stone, often character, and rarely of any classificatory value. On the other known as its " fire,” is due to the decomposition of the white hand, a grouping based only on colour led to several names being
light which enters the stone, and is returned, by internal in some cases applied to the same mineral species. Thus the Disper
reflection, after resolution intoits coloured components. ruby and sapphire are essentially identical in chemical composi
This decomposition depends on the dispersive power tion and in all physical characters, save colour. of the substance. The exceptional beauty of the fiery flashes Descriptions of precious stones by ancient writers generally are in the diamond is due to its high dispersion, in other words, to too vague for exact diagnosis. The principal classical authorities the difference between the refractive indices for the red rays and are Theophrastus and the elder Pliny. Stones were the violet rays at the extremities of the spectrum. The peculiar formerly held in esteem not only for their beauty and
Superstia lustre exhibited by the diamond is called adamantine, and is rarity but for the medicinal and magical powers with shared to some extent by certain other stones which have a which they were reputed to be endowed. Up to comparatively high refractive index and high dispersion, such as zircon. recent years the toadstone, for example, was worn not for beauty
The use of the spectroscope may be valuable in discriminating but for sake of occult virtue; and even at the present day between certain precious stones. It was shown by Sir A. H. certain stones, like jade, are valued for a similar reason. Prof. Spectro
Church' that almandine garnet and zircon when simply W. Ridgeway has suggested that jewelry took its origin not, as scopie
viewed through this instrument give, under proper often supposed, in an innate love of personal decoration, but conditions, characteristic absorption spectra, due to rather in the belief that the objects used possessed magical virtue.
the light reflected from the stone having penetrated Small stones peculiar in colour or shape, especially those with to some extent into the substance of the mineral and suffered I natural perforations, are usually valued by 'uncivilized peoples
as amulets. The Orphic poem Atbirá, reputed to be of very early , fortunately the relief is incomplete, and the published illustra. though unknown date, is rich in allusions to the virtues of many lion inadequate. It would scem, 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 haenor- Both systems are clearly indicated by Pliny, who in one passage rhage; a green jasper would bring ser:ility to the soil; and the (H.N. xxxvii. 60) states that diamond splinters are sought out by purple wine-colour of amethyst pointed io 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 any degree of hardness; while elsewhere (H.N. xxxvii. 200) he to modern times, and even at the present day the belief in “lucky speaks of the special efficacy of the scrvor lerebrarum, the vehement stones” is by no means extinct.
action of drills. A third method is also indicated by Pliny (ibid.) BIBLIOGRAPHY.—The most comprehensive work on gçm-stones is when he speaks of the use of a blunted tool, which must have been Professor Max Bauer's Edelsteinkunde (1896). translated;, with moistened and supplied with emery of Naxos. additions, by L. J. Spencer under the title Precious Stones (1904). Less detailed are Professor P. Groth's Grundriss der Edelsteinkunde
A four-sided pendant of the Hellenistic period published by (1887) and Professor C. Doclter's Edelsteinkunde (1893). Sir A. Furtwängler (Antike Gommen, Gesch. p. 400) shows clearly the H. Church's Precious Stones (1905), intended as a guide to the successive stages of the operation. On side a the subject is collections in the Victoria and Albert Museum, is a convenient slightly sketched in with the diamond point. On side b the introduction; and Professor H. A. Miers's Cantor Lectures at the Society of Arts on Precious Stones (1896) may be studied with deepest parts of the figure have also been roughly scooped out advantage. For American stoncs, the valuable work of Dr G. F. with the wheel. On sides c and d the wheel work is fairly comKunz, The Gems and Precious Stones of N. America, is a standard plete, but the finer internal work has not been begun. authority; and the Annual Reports of this writer and others, published by the Geological Survey of the United States in the received a final polish on its surface, to obliterate any erroncous,
After the design had been completed the stone must have Mineral Resources, form a repertory or valuable information on precious stones in general. The articles in The Mineral Industry strokes of the first sketch; but this process was not carried as far (founded by R. P. Rothwell) should also be consulted. See likewise as in modern work. It is a popular error to suppose that a high o. C. Farrington, Gems and Gem Minerals (Chicago, 1903); For degree of internal polish is a proof of antiquity. If the interior of Herbert Smilk Refractometer (London, 1907); L. Claremont, The Gem- the design has a high degree of polish it may be either ancient or Culler's Craft (London, 1906); W. Goodchild, Precious Stones modern, or it may be an ancient stone repolished in modern times. (London, 1908).
(F. W. R.") If it has a matt surface uniformly produced by intention, it is 2. GEMS IN ART
probably modern. If the design is slightly dimmed and worn or
scratched the stone may be antique, but is not necessarily so, In art, the word Gem is the general term for precious stones since modern engravers have observed this peculiarity, and have when engraved with designs, whether adapted for sealing(oopayis, imitated it with a success which, were there no other grounds of sigillum, intaglio), or mainly for artistic effect (imagines eclypae, suspicion, might escape detection. cameo). They exist in a very large number of undoubtedly History.-It has been a subject of controversy whether the genuine old examples, extending from the mists of Babylonian first infancy of the art was passed in Egypt or in Babylonia, but antiquity to the decline of Roman civilization, and again starting it seems highly probable that it was developed in Babylonia, with a new, but less original impulse on the revival of art. Apart whence at any rate the oldest examples of engraved gems at from workmanship they possess the charms of colour deep, rich, present known are obtained. It does not necessarily follow, and varied, of material unequalled for its endurance, and of however, that Egypt was therefore a pupil. It may well be that scarcity, which in many instances has been enhanced by the the art was developed independently in the two countries, although remoteness of the lands whence they came or the fortuity of their certain points of possible contact in respect of the forins employed occurrence. These qualities united within the small compass of will be described below in the section dealing with primitive a gem were precisely such as were required in a seal as a thing Egypt. of constant use, so inalienable in its possession as to become Babylonio.-At a very remote period the cylindrical form of naturally a personal ornament and an attractive medium of stone was introduced and became the approved shape, while the artistic skill, no less than the centre of traditions or of religious technical skill of the artist was still slight, and the iraces of the and legendary associations. As regards the nations of classical tools employed (drill and pencil point) were still unconcealed. antiquity, all seals are classed as gems, though in many cases the The cylinder was suspended by a string and used as a seal. material is not such as would strictly come under that heading, Impressions of cylinders are frequent on contract tablets. If one and precious stones in the modern sense are hardly known to of the parties cannot use a seal he makes a nail-mark in lieu occur. On the other hand it must not be supposed that gems thereof, as is recorded in the document. engraved in intaglio were necessarily employed as seals. At all But from a time that was still comparatively early the en. periods many intaglios are found which could not have been so gravers could work with considerable skill in the hard stone. In employed without great difficulty. In Greece and Rome, within particular a cylinder may be quoted in the de Clercq Collection historic times, gems were worn engraved with designs to show bearing the name of Sargon I. of Agade, who is placed about that the bearer was an adherent of a particular worship, the 3500 B.C. The cylinder is engraved with the king's name and follower of a certain philosopher, or the attached subject of an titles and two symmetrically disposed renderings of Izdubar, with emperor. However, speaking generally, the intaglio engraving is a vase of flowing water giving drink to a bull. The whole is a means to an end, namely, a seal-impression, while an engraving treated in a conventionalized style that indicates long traditions. in relief is complete in itself.
An important early cylinder in the British Museum is inscribed Melhods of Engraving (see also under LAPIDARY).-In gem- with the name of a viceroy of Ur-Gur, king of Ur (about 2500 B.c.) engraving the principal modern implement is a wheel or minute The engraving shows Ur-Gur being led into the presence of Sin, copper disk, driven in the manner of a lathe, and moistened with the moon-god. olive oil mixed with emery or diamond dust. There is no clear The cylinder seal was adopted by the Assyrians, and so was proof of the use among the ancients of a wheel mounted lathe carried on continuously till the time of the Persian conquest of wise, but we have abundant indications of drilling with a revolving Babylon (538 B.c.). Meanwhile, as an alternative form the tool, which might be either a tubular drill making a ring-like conoidal seal, rounded at the top and having a flat base for the depression, a pointed tool making a cup-like sinking, or a small intaglio, came into use beside the cylinder, wheel with a cutting edge, making a boat-shaped depression. In style the Assyrians carried on the Babylonian tradition, but
We have onc sepulchral monument from Philadelphia show with no freedom of design. Subjects and treatment became ing the tool of an intaglio engraver (OAKTU OKOLloyudos; see rigidly conventional. Alkenische Milleilungen des Arch. Inst. xv. p. 333). Un After the Persian conquest the victors adopted the cylinder