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For the great majority of signed gems which do not fall into | Byzantine empire down to nearly the epoch of the Renaissance. these categories the reader must refer to the discussions of From the Byzantine period downward one peculiarty of gemFurtwängler and others (see Bibliography below). It must engraving becomes noticeable. Cameo-work as compared with suffice to say that Furtwängler aèrives at the result that we have intaglios in classical times was rare and infrequent, but now and in all genuine signatures of at least fifty ancient gem-engravers. onwards the opposite is the case, intaglio-sinking having almost
Gem-Engraving in the Later Empire. - In the following centuries died out, and cameos being chiefly produced. Commercial the art of intaglio engraving, which was still at a high degree of intercourse with the East still secured for the engravers a supply perfection in the first century of the Roman empire, became of magnificent sardonyxes, although blood-stone and other more mechanical. The designs have a very characteristic ap- non-banded stones were very commonly used for works in relief. pearance, due to the method of production with rough and hasty Cameos during the long dark ages were used chiefly for the decora. strokes of wheel only. A collection of gems found in England, tion of reliquaries and other altar furniture, and as such their such as that in the possession of the corporation of Bath, shows designs were purely ecclesiastical or scriptural. To this period the feeble character in particular of the gems current in the also belongs the class of complimentary or motto cameos, which, provinces. Except in portraiture, and in grylli or conceits, in containing only inscriptions and an ornamental border, executed which various things are combined into one, often with much in nicolo stones, were used as personal gifts and adornments. skill, the subjects were as a rule only variations or adaptations In medieval times antique camcos were held in peculiar veneraof old types handed down from the Greeks. When new and tion on account of the belief, then universal, in their potency distinctly Roman subjects occur, such as the finding of the head as medicinal charms. This power was supposed to be derived on the Capitol, or Faustulus, or the she-wolf with the twins, from their origin, of which two theories, equally satisfactory, both the stones and the workmanship are poor. In such cases, were current. By the one they were held to be the work of the where the design stirs a genuine national interest, it may happen children of Israel during their sojourn in the wilderness (hence that very little of artistic rendering will be acceptable rather than the name Pierres d'Israël), while the other theory hold them to otherwise, and much more is this true when the design is a symbol be direct products of nature, the engraved figures pointing to of some article of faith, as in the early Christian gems. There the peculiar virtue lodged in them. Interpreters less mystically both the art and the material are at what may be called the lowest inclined found Biblical interpretations for the subjects. Thus Jevel. The usual subjects on the early Christian gems are the the cameo of the Sainte Chapelle was supposed to represent the fish, anchor, ship, dove, the good shepherd, and, according to triumph of Joseph in Egypt. A cameo with Poseidon, Athena
and her serpent was Adam and Eve.
The revival of the glyptic arts in western Europe dates from the pontificate of the Venetian Paul II. (1464-1471), himself an ardent lover and collector of gems, to which passion, indeed, it is gravely affirmed he was a martyr, having died of a cold caught by the multiplicity of gems exposed on his fingers. The cameos of the early part of the 16th century rival in beauty of execution the finest classical works, and, indeed, many of them
pass in the cabinets of collectors for genuine antiques, which 19.90
they closely imitated. The Oriental sardonyx was not available
for the purposes of the Renaissance artists, who were conse. FIG. 11.- Chris. Fig. 12.-Gnostic FIG. 13.-Sassanian quently obliged to content themselves with the colder German tian Gem. The Good Gem. (Brit. Mus.) Gem. (Brit. Mus.) agate onyx. The scarcity of worthy materials led them to use Shepherd. (Brit.Mus.)
the backs of ancient camcos, or to improve on classical works of
inferior value executed on good material, and probably to this Clemens, the lyre. Under the Gnostics, however, with whom cause must also be assigned the development of shell cameos, there was more of speculation than of faith, symbolism was which are rarely found, of an older period. developed to an extent which no art could realize without the Among the means of distinguishing antique cameos from aid of writing. A gem was to them a talisman more or less cinquecento work, the kind of stone is one of the best tests, the elaborate with long, but for the most part quite unintelligible, classical artists having used only rich and warm-tinted Oriental engraved formulae. The difficulty is to make out how the stones stones, which further are frequently drilled through their diawere carried; many specimens exist, but none show signs of meter with a minute hole, from having been used by their original mounting. The materials are usually haematite or jasper. As Oriental possessors in the form of beads. The cinquecento artists regards the designs, it is clear that Egyptian sources have been also, as a rule, worked their subjects in high relief, and resorted most drawn upon. But the symbolism is also largely associated to undercutting, no case of which is found in the flat low work with Mithraic worship. The name Abraxas, or more correctly of classical times. The projecting portions of antique work Abrasax, which, from its frequency on these gems, has led to exhibit a dull chalky appearance, which, their being called also “Abraxas gems," is, when the Greek however, fabricators learned to imitatc letters of which it is composed are treated as Greek numerals, in various ways, one of which was by equal to 365, the number of days in a year, and the same is the cramming the gizzards of turkey fowls case with MEIOPAZ.
with the gems. Another index of anMore interesting, from the occasionally forcible portraiture tiquity is found in the different methods and the splendour of some of the jacinths employed, are the of working adopted in classical and Sassanian gems, which as a class may be said io represent the Renaissance times. The tools employed last stage of true gem-engraving in ancient times.
by the Renaissance engraver were the The art of cameo-engraving, which, as we have seen, attained drill and the wheel, while the ancient its greatest splendour at the beginning of the empire, followed on artist also employed the diamond point. the whole a similar course. It waned in the early part of the The gem-engraver's art again during 3rd century after the death of the emperor Severus, but under the 18th century revived under an even
QIAXIA the first Christian emperor Constantine it enjoyed a brief period greater amount of encouragement from of revival. Fine cameo portraits of Constantine are extant; men of wealth and rank. In this last
Fig. 14-Muse, by and it was during or shortly after his reign that Christian period the names of engravers who
Pichler. (Brit. Mus.) Scripture subjects began to appear on cameos. That class of succeeded best in imitating classical designs were Natter, subjects constituted the staple of such work-generally rude Pichler (fig. 14), and the Englishmen Marchant (fig. 15) and and artistically debased-as continued to be cultivated under the Burch. Compared with Greek gems, it will be seen that what
For the history : P:
at first sight is attractive as refined and delicate is after all an Special Periods:-Babylonia, &c. --Menant,." Les Pierres gravées exaggerated minuteness of execution, entirely devoid of the de la baute Asie," Recherches sur lo glyplique orientale (1883–1886). ancient spirit. The success with which modern engravers imposed Tombs of the First Dynasty" (Egypi Explor. Fund, XVIIlth
Egypl.--For the early cylinder sealings, &c., see Petrie, "Royal on collectors is recorded in many instances, of which one may be Memoir), p. 24: pls. 12, figs. 3 to 7, and pls. 18-29; Amélineau,
taken as an instructive “Nouvelles Fouilles d'Abydos, 1897–1898," Comple rendu, pp. 78. type. In the Biblio- 423he Bible. Petrie, “Stones (Precious).” in Hastings' Dict. of the thèque Nationale is a
Bible. gem (Chabouillet'scata Phoenician.-See M. A. Levy, Siegel und Gemmen, with three logue, No. 2337), famil- plates of gems having Phoenician, Aramaic, old Hebrew and other iarly known as the inscriptioris (Breslau, 1869): and, on the same subject, De Vogue,
in the Revue archéologique, 2nd series (1868), xvii. p. 432, pls. 14-16. signet of Michelangelo, Crete.- Articles by A. J. Evans in Journal of Hellenic Studies, xiv., the subject being a xvii., xxi., and in Annual of British School al Athens, vi. and onwards.
Bacchanalian scene. So Classical Gems.-See Furtwängler, op. cil. FIG. 15.-Nereid and Sea-bull by
much did he admire it, - Abrasax."
Gnostic Gems.-Cabrol, Dict. d'archéologie chrétienne, s.v. Marchant. (Brit. Mus.)
the story says, that he
For the controversy as to gems with artists' signatures, see copied from it one of the groups in his paintings in the Sistine Koehler, Abhandlung über die geschnittenen Steine, mil den Namen chapel. The gem, however, is evidently in this part of it a mere der Künstler; Koehler's collected works, ed. Stephani, vol. iii. copy from Michelangelo's group, and therefore a subsequent (1857); Stephani, Notes to Kochler as above; also Uber einige
angebliche Steinschneider des Allerthums (St Petersburg, 1851); production, probably by da Pescia. In our own day the engraving of cameos has practically ceased Furtwängler, Jahrbuch d. k. deutsch, arch. Inst. i. (1888), pp. 105,
Brunn, Geschichte der griechischen Künstler, ii. (1859), pp. 442-637; to be pursued as an art. Roman manufacturers cut stones in 193, 297; iv. (1889), p. 46, and Geschichte, passim. large quantities to be used as shirt-studs and for setting in finger
the Poniatowski gems, see Reinach, Pierres rings, and in Rome and Paris an extensive trade is carried on in gravées, p. 151.
Catalogues.-The chief catalogues dealing with modern public the cutting of shell cameos, which are largely imported into collections are: Berlin, A. Furtwängler, Beschreibung der geEngland and mounted as brooches by Birmingham jewelry schnittenen Steine im Antiquarium (1896); British Museum, A. H. manufacturers. The principal shell used is the large bull's. Smith, A Catalogue of Engraved Gems in the British Museum (Dept. mouth shell (Cassis rusa), found in East Indian scas, which has R. Greek and Roman Antiquities) (1888): Paris, Bibliothèque
des camées et pierres gravées a sard-like underlayer. The black helmet (Cassis tuberosa) of de la Bibliothèque Impériale (1858), E. Babelon, ‘Catalogue des the West Indian seas, the horned helmet (C. cornula) of Mada- camées ... de la Bibliothèque Nationale (1897). gascar, and the pinky queen's conch (Strombus gigas) of the
Modern Engraving.-Vasari vii, p. 113. (ed. Siena, 1792); conWest Indics are also employed. The famous potter Josiah older books on gems are very numerous, but those of present-day
tinued by Mariette, Traité des pierres gravées (1750), i. p. 105. The Wedgwood introduced a method of making imitations of cameos importance are not many. Faber, Illustrium imagines apud in pottery by producing white figures on a coloured ground, Fulvium Ursinum (Antwerp, 1606); Stosch, Gemmae antiquae this constituting the peculiarity of what is now known as Winckelmann, Description des pierres gravées du feu Baron de Stosch Wedgwood ware. Gem Collectors.-The habit of gem-collecting is recorded first a convenient reissue of Stosch, and seven others of the older works,
(1760); Krause, Pyrgoteles, oder die edlen Steine der Allen (1856); in the instance of Ismenias, a musician of Cyprus, who appears by S. Reinach, Pierres gravées, &c. . . . réunies et réédilées, avec to have lived in the 4th century B.C. But though individual un texte nouveau (1895). collectors are not again mentioned till the time of Mithradates, gems was that issued by James Tassie
of Glasgow, witb A Descriptive
Pastes. The principal collection of glass and sulphur pastes from whose cabinet was carried off to Rome by Pompey, still it is to Catalogue of a General Collection of : be inferred that they existed, if not pretty generally, yet in such arranged and described by R. E. Raspe (the author of Baron Mun. places as Cyrene, where the passion for gems was so great that chausen) (1791).
(A.S. M.; A. H. Sm.) the thriftiest person owned one worth 10 minas, and where, GEM, ARTIFICIAL. The term “ Artificial Gems" does not according to Aclian (Var. hist. xii. 30), the skill in engraving mean imitations of real gems, but the actual formation by artiwas astonishing. The first cabinet (dactyliotheca) in Rome was ficial means of the real precious stone, so that the product is that of Scaurus, a stepson of Sulla. Caesar is said to have formed identical, chemically, physically and optically, with the one six cabinets for public exhibition, and from the time of Augustus found in nature. For instance, in chemical composition the all men of refinement were supposed to be judges both of the art lustrous diamond is nothing but crystallized carbon. Could we and of the quality of the stones.
take black amorphous carbon in the form of charcoal or lampIn the middle ages the chief collections were incorporated in black and dissolve it in a liquid, and by the slow evaporation of works of art in the church treasurics. The first collector of that liquid allow the dissolved carbon to separate out, it would modern times was, as already mentioned, Pope Paul II., who was probably crystallize in the transparent form of diamond. This followed by a long succession of princely and noble collectors such would be a true synthesis of diamond, and the product would be as Lorenzo de' Medici and the great earl of Arundel. The col- just as much entitled to the name as the choicest products of lection of the latter passed into the hands of the dukes of Marl- Kimberley or Golconda. But this is a very different thing from borough and thence into the possession of Mr David Bromilow. the imitation diamond so common in shop windows. Here the The collection was finally dispersed by auction in June 1899. chemist has only succeeded in making a paste or glass having
In modern times the principal collections are contained in state limpidity and a somewhat high refractivity, but wanting the muscums. The cabinets of Vienna and of the Bibliothèque hardness and "fire" of the real stone. Nationale are incomparably rich in the historic cameos. Those The Diamond.-Within recent years chemists have actually of the British Muscum and of Berlin are the strongest in their succeeded in making the real diamond by artificial means, and range over the whole field of the gem-engraver's art.
although the largest yet made is not more than one-fiftieth of BIBLIOGRAPHY.-For the fullest general account of the subject an inch across, the process itself and the train of reasoning leading (with especial attention to the gems of classical antiquity), see A. up to such an achievement are sufficiently interesting to warrant Furtwängler, Die antiken Gemmen, Geschichte der Steinschneiderkunst a somewhat full description. Attempts to make diamonds Gravure en pierres fines, camées e intailles (1894): A. H. Smith, artificially have been numerous, but, with the sole exception of Gemma" and " Sculptura," in the 3rd edition of Smith's Dict. of those of Henri Moissan, all have resulted in failure. The nearest Antiquities; J. H. Middleton, The Engraved Gems of Classical Time's approach to success was attained by J. B. Hannay in 1880 and (1891). Much curious information is in the works of C. W. King: R.'S. Marsden in 1881; but their results have not been verified Nalural History. Ancient
and Modern; of Precious Stones and Gems, by others who have tried to repeat them, and the probability and of the Precious Metals (1865); Anligue Gems and Rings (2 vols., is that what was then thought to be diamond was in reality 1872).
carborundum or carbide of silicon.
Attempts have been made by two methods to make carbon diamonds--carbonado, in fact-and a small quantity of transcrystallize in the transparent form. One is to crystallize it slowly parent colourless diamonds showing crystalline structure. from a solution in which it has been dissolved. The difficulty is Besides these there may be corundum and carbide of silicon, to find a solvent. Many organic and some inorganic bodies hold arising from impurities in the materials employed. Heating carbon so loosely combined that it can be separated out under the with strong sylphuric acid, with hydrofluoric acid, with nitric influence of chemical action, heat or electricity, but invariably acid and potassium chlorate, and fusing with potassium fluoridethe carbon assumes the black amorphous form. The other operations repeated over and over again-at last eliminate the method is to try to fuse the carbon by fierce heat, when from graphite and impurities and leave the truc diamond untouched. analogy it is argued that on cooling it will solidify to a clear limpid | The precious residue on microscopic examination shows many crystal. The progress of science in other directions has now pieces of black diamond, and other colourless transparent pieces, made it pretty certain that the true mode of making diamond some amorphous, others crystalline. Although many fragments artificially is by a combination of these two methods. Until of crystals are seen, the writer has scarcely ever met with a recently it was assumed that carbon was non-volatile at any complete crystal. All appear broken up, as if, on being liberated attainable temperature, but it is now known that at a tempera- from the intense pressure under which they were formed, they ture of about 3600° C. it volatilizes readily, passing without burst asunder. Direct evidence of this phenomenon has been liquefying directly from the solid to the gaseous state. Very few seen. A very fine piece of diamond, prepared in the way just bodies act in this manner, the great majority when heated at described and carefully mounted on a microscopic slide, exploded atmospheric pressure to a sufficient temperature passing through during the night and covered the slide with fragments. This the intermediate condition of liquidity. Some few, however, bursting paroxysm is not unknown at the Kimberley mines. which when heated at atmospheric pressure do not liquefy, when Şir William Crookes in 1906 communicated to the Royal heated at higher pressures in closed vessels obey the common rule Society a paper on a new formation of diamond. Sir Andrew and first become liquid and then volatilize. Sir James Dewar Noble has shown that in the explosion of cordite in closed steel found the critical pressure of carbon to be about 15 tons on the cylinders pressures of over 50 tons to the sq. in. and a temperature sq. in.; that is to say, if heated to its critical temperature (3600° probably reaching 5400° were obtained. Here then we have C.), and at the same time subjected to a pressure of 15 tons to conditions favourable for the liquefaction of carbon, and if the the sq. in., it will assume the liquid form. Enormous as such time of explosion were sufficient to allow the reactions to take pressures and temperatures may appear to be, they have been place we should expect to get liquid carbon solidified in the exceeded in some of Sir Andrew Noble's and Sir F. Abel's re- crystalline state. Experiment proved the truth of these anticipasearches; in their investigations on the gases from gunpowder tions. Working with specially prepared explosive containing a and cordite fired in closed steel chambers, these chemists ob- little excess of carbon Sir Andrew Noble collected the residue tained pressures as great as 95 tons to the sq. in., and temperatures left in the steel cylinder. This residue was submitted by Sir as high as 4000° C. Here then, if the observations are correct, William Crookes to the lengthy operations already described we have sufficient temperature and enough pressure to liquely in the account of H. Moissan's fused iron experiment. Finally, carbon; and, were there only sufficient time for these to act on minute crystals were obtained which showed octahedral planes the carbon, there is little doubt that the artificial formation of with dark boundaries due to high refracting index. The position diamonds would soon pass from the microscopic stage to a scale and angles of their faces, and cleavages, the absence of bimore likely to satisfy the requirements of science, if not those refringence, and their high refractive index all showed that the of personal adornment.
crystals were true diamond. It has long been known that the metal iron in a molten state The artificial diamonds, so far, have not been larger than dissolves carbon and deposits it on cooling as black opaque microscopic specimens, and none has measured more than about graphite. Moissan carried out a laborious and systematic series half a millimetre across. That, however, is quite enough to show of experiments on the solubility of carbon in iron and other the correctness of the train of reasoning leading up to the achieve mctals, and came to the conclusion that whereas at ordinary ment, and there is no reason to doubt that, working on a larger pressures the carbon separates from the solidifying iron in the scale, larger diamonds will result. Diamonds so made burn in form of graphite, if the pressure be greatly increased the carbon the air when heated to a high temperature, with formation of on separation will form liquid drops, which on solidifying will carbonic acid; and in lustre, crystalline form, optical properties, assume the crystalline shape and become true diamond. Many density and hardness, they are identical with the natural stone. other metals dissolve carbon, but molten iron has been found to It having been shown that diamond is formed by the separation be the best solvent. The quantity entering into solution increases of carbon from molten iron under pressure, it became of interest with the temperature of the metal. But temperature alone is not to see if in some large metallurgical operations similar conditions enough; pressure must be superadded. Here Moissan ingeniously might not prevail. A special form of steel is made at some made use of a property which molten iron possesses in common large establishments by cooling the molten metal under intense with some few other liquids-water, for instance of increasing hydraulic pressure. In some samples of the steel so made in volume in the act of passing from the liquid to the solid state. Professor Rosel, of the university of Bern, has found microscopic Pure iron is mixed with carbon obtained from the calcination of diamonds. The higher the temperature at which the steel has sugar, and the whole is rapidly heated in a carbon crucible in an been melted the more diamonds it contains, and it has even been electric furnace, using a current of 700 amperes and 40 volts. The suggested that the hardness of steel in some measure may be iron melts like wax and saturates itself with carbon. After a few due to the carbon distributed throughout its mass being in this minutes' heating to a temperature above 4000° C.- a tempora- adamantine form. The largest artificial diamond yet formed ture at which the lime furnace begins to melt and the iron was found in a block of steel and slag from a furnace in Luxemvolatilizes in clouds—the dazzling, fiery crucible is lifted out and bourg; it is clear and crystalline, and measures about one-fiftieth plunged beneath the surface of cold water, where it is held till it of an inch across. sinks below a red heat. The sudden cooling solidifies the outer A striking confirmation of the theory that natural diamonds skin of molten metal and holds the inner liquid mass in an iron have been produced from their solution in masses of molten grip. The expansion of the inner liquid on solidifying produces iron, the metal from which has gradually oxidized and been enormous pressure, and under this stress the dissolved carbon washed away under cycles of atmospheric influences, is afforded separates out in a hard, transparent, dense form-in fact, as by the occurrence of diamonds in a meteorite. On a broad open diamond. The succeeding operations are long and tedious. plain in Arizona, over an area of about 5 m. in diameter, lie The metallic ingot is attacked with hot aqua regia till no iron is scattered thousands of masses of metallic irony. the fragments left undissolved. The bulky residue consists chiefly of graphite, varying in weight from half a ton to a fraction of an ounce. There together with translucent flakes of chestnut-coloured carbon, is little doubt that these fragments formed part of a meteoric hard black opaque carbon of a density of from 3.0 to 3.5, black ) shower, although no record exists as to when the fall took place,
Near the centre, where most of the fragments have been found, but the use of borax made the necessary difference. But it was is a crater with raised edges, thrce-quarters of a milc in diameter not till about the year 1877 that E. Frémy and C. Feil first and 600 ft. deep, bearing just the appearance which would be published a method whereby it was possible to produce a crys. produced had a mighty mass of iron-a falling star-struck the tallized alumina from which small stones could be cut. They ground, scattered it in all directions, and buried itself decply first formed lead aluminate by the fusion together of lead oxide under the surface, fragments croded from the surface forming and alumina. This was kept in a state of fusion in a fireclay the pieces now met with. Altogether ten tons of this iron have crucible (in the composition of which silica enters largely). been collected, and specimens of the Canyon Diablo meteorite Under the influence of the high temperature the silica of the are in most collectors cabincts. Dr A. E. Foote, a mincralogist, crucible gradually decomposes the lead aluminate, forming lead when cutting a section of this metcorite, found the tools injured silicate, which remains in the liquid state, and alumina, which by something vastly harder than metallic iron, and an emery crystallizes as white sapphire. By the admixture of 2 or 3% wheel used for grinding it was ruined. He attacked the specimen of a chromium compound with original materials the resulting chemically, and soon afterwards announced to the scientific white sapphire became ruby. More recently Edmond Frémy world that the Canyon Diablo meteorite contained diamonds, and A. Verncuil obtained artificial rubies by reacting at a red both black and transparent. This startling discovery was heat with barium fluoride on amorphous alumina containing subsequently verified by Professors C. Friedel and H. Moissan, a small quantity of chromium. The rubics obtained in this and also by Sir W. Crookes.
manner are thus described by Frémy and Verneuil: “ Their The Ruby:- It is evident that of the other precious stones only crystalline form is regular; their lustre is adamantine; they the most prized are worth producing artificially. Apart from present the beautiful colour of the ruby; they are perfectly their inferior hardness and colour, the demand for what are transparent, have the hardness of the ruby, and easily scratch known as “semi-precious stones” would not pay for the topaz. They resemble the natural ruby in becoming dark when necessarily great expenses of the factory. Morcover, were it to heated, resuming their rose-colour on cooling." Des Cloizeaux be known that they were being produced artificially the demand-says of them that " under the microscope some of the crystals never very great-would almost ccase. The only other gems, show bubbles. In converging polarized light the coloured rings therefore, which nced be mentioned in connexion with their and the negative black cross are of a remarkable regularity." artificial formation are those of the corundum or sapphire class, Other experimentalists have attacked the problem in other which include all the most highly prized gems, rivalling, and directions. Besides those already mentioned, L. Elsner, H. H. De sometimes exceeding, the diamond in value. Here a remarkable Senarmont, Sainte-Claire Deville, and H Caron and H. Debray and little-known fact deserves notice. Excepting the diamond have succeeded with more or less success in producing rubies. and sapphire, each of the precious stones—thc emerald, the The general plan adopted has been to form a mixture of salts topaz and amethyst-possesses a more noble, a harder, and fusible at a red heat, forming a liquid in which alumina will more highly-prized counterpart of itself, alike in colour, but dissolve. Alumina is now added till the fused mass will take up superior in brilliancy and hardness; still more strange, the no more, and the crucible is left in the furnace for a long time, precious stone to which its special name is usually attached sometimes extending over weeks. The solvent slowly volatilizes, is the variety the least prized. The ruby itself might almost and the alumina is deposited in crystals, coloured by whatever be included in the same category. The true ruby consists of colouring oxide has been added. the carth alumina, in a clear, crystalline form, having a minute Mention has been made above of a stone frequently substituted quantity of the element chromium as the colouring matter. It for the true ruby, called the "spinel " or "balas” ruby. The is often called the “Oriental Ruby," or red sapphire, and when spinel and ruby occur together in nature, stones from Burma of a paler colour, the “ Pink Sapphire.” But the ruby as met being as often spinel as true Oriental ruby. In the artificial with in jewellers' shops of inferior standing is usually no true production of the ruby it sometimes happens that spinel crystalruby, but a "spinel ruby" or " balas ruby," sometimes very lizes out when true Oriental ruby is expected. The fusion bath beautiful in colour, but softer than the Oriental ruby, and is so arranged that only red-coloured alumina shall crystallize out, different in chemical composition, consisting essentially of alumina but it is difficult to have all the materials of such purity as to and magnesia and a little silica, with the colouring matter ensure the complete absence of silica and magnesia. In this chromium. The colourless basis of the true Oriental precious case, when these impurities have accumulated to a certain point stones being taken as crystallized alumina or white sapphire, they unite with the alumina, and spinel then separates, as it when the colouring matter is red the stone is called ruby, when crystallizes more easily than ruby. When all the magnesia and blue sapphire, when green Oriental emerald, when orange-yellow silica have been eliminated in this way the bath resumes its Oriental topaz, and when violet Oriental amethyst. Clear, deposition of crystalline ruby. Rubies of fine colour and of colourless crystals are known as white sapphire, and are very considerable size have been shown in London, made on the valuable. it is evident, therefore, that whosoever succeeds in Continent by a secret process. The writer has seen several cut making artificially clear crystals of white sapphire has the stones so made weighing over a carat each, the uncut crystals power, by introducing appropriate colouring matter, to make measuring half an inch along a crystal edge, and weighing over the Oriental ruby, sapphire, emerald, topaz and amethyst. All 70 grains, and a clear plate of ruby cut from a single crystal of these stones, even when of small size, are costly and readily weighing over 10 grains. Ruby has been made by Sir w. saleable, while when they are of fine quality and large size they Roberts-Austen as a by-product in the production of metallic are highly prized, a ruby of fine colour, and free from flaws, a chromium. Oxide of chromium and aluminium powder are few carats in weight, being of more value than a diamond of intimately mixed together in a refractory crucible, and the the same weight.
mixture is ignited at the upper part. The aluminium and This being the case, it is not surprising that repeated attempts chromium oxide react with evolution of so much heat that the have been made to effect the crystallization of alumina. This reduced chromium is melted. Such is the intensity of the reaction is not a matter of difficulty, but unfortunately the crystals that the resulting alumina is also completely fused, floating as a generally form thin plates, of good colour, but too thin to be liquid on the molten chromium. Sometimes the alumina takes useful as gems. In 1837 M. A. A. Gaudin made true rubies, of up the right amount of chromium to enable it to assume the ruby microscopic size, by fusing alum in a carbon crucible at a very colour. On cooling the melted alumina crystallizes in large high temperature, and adding a little chromium as colouring fakes, which on examination by transmitted light are seen to be matter. In 1847 J. J. Ebelmen produced the wbite sapphire true ruby. The development of the red colour is said by C. and rose-coloured spinel by fusing the constituents at a high Greville-Williams only to take place at a white heat. It is not due temperature in boracic acid. Shortly afterwards he produced to the presence of chromic acid, but to a reaction between alumina the ruby by employing borax as the solvent. The boracic acid and chromic oxide, which requires an elevated temperature. was found to be too volatile to allow the alumina to crystallize, Artificially made but real rubies have been put on the market;
prepared by a process of fusion by A. Verneuil. He finds that I coloured with a small quantity of ferric oxide. It has been certain conditions have to be fulfilled in order to get the alumina in produced artificially by adding iron instead of chromium to the a transparent form. The temperature must not be higher than is matrix from which the white sapphire crystallizes. absolutely necessary for fusion. The melted product must always The Zircon.—The zircon is a very beautiful stone, varying in be in the same part of the oxyhydrogen flame, and the point of colour, like the topaz, from red and yellow to green and blue. contact between the melted product and the support should be it is sometimes met with colourless, and such are its refractive reduced to as small an area as possible. M. Verneuil uses a powers and brilliancy that it has been mistaken for diamond. vertical blowpipe flame directed on a support capable of move. It is a compound of silica and zirconia. H. Sainte-Claire Deville ment up and down by means of a screw, so that the fused product formed the zircon artificially by passing silicon fluoride at a red may be removed from the zone of fusion as it gets higher by heat over the oxide zirconia in a porcelain tube. Octahedral addition of fresh material. The material employed is either crystals of zircon arc then produced, which have the same composed of small, valueless rubics, or alumina coloured with the crystalline form, appearance and optical qualities as the natural right amount of chromium. It is very finely powdered and fed in zircon. through the blowpipe orifice, whence it is blown in a highly BIBLIOGRAPHY.-Sir William Crookcs, “A New Formation of heated condition into the zone of fusion. The support is a small Diamond,". Proc. Roy. Soc. vol. lxxvi. P. 458; “ Diamonds," a cylinder of alumina placed in the axis of the blowpipe. As the lecture delivered before the British Association at Kimberley, operation proceeds the finc grains of powder driven on to the 135, 147, 1597). J. Ebelmen, "Sur la production artificielle des support in the zone of fusion form a cone which gradually rises pierres dures," Comples rendus, vol. xxv. P: 279; “Sur une nouvelle and broadens out until it becomes of sufficient size to be used for méthode pour obtenir, par la voie sèche, des combinations crystalcutting. Rubies prepared in this way bave the same specific lisées, et sur ses applications à la réproduction de plusieurs espèces gravity and hardness as the natural ruby, and they are also C. Feil, “Sur la production artificielle du corindon, du rubis, et de dichroic, and in the vacuum tube under the influence of the différents silicates crystallisées," Comples rendus, vol. lxxxv: P. cathode stream they phosphoresce with a discontinuous spectrum 1029; C. Friedel,“ Sur l'existence du diamant dans le fer météorique showing the strong alumina linc in the red. When properly cut
de Cañon Diablo," Comptes rendus, vol. cxv. p. 1037, vol. cxvi. and mounted it is almost impossible to distinguish them from P: 290: H. Moissan, “Etude de la météorite de Cañon Diablo." natural stones.
Comples rendus, vol. cxvi. P: 288; - Expériences sur la réproduction
du diamant," Comples rendus, vol. cxviii. p. 320; Sur quelques The Sapphire.-Auguste Daubrée has shown that when a full expériences relatives à la préparation du diamant," Comptes rendus, quantity of chromium is added to the bath from which white vol. cxxiii. p. 206; Le Four électrique (Paris, 1897); H. Sainte-Claire sapphire crystallizes the colour is that of ruby, but when much Deville and H. Caron, “. Sur un nouvсau mode de production à
l'état cristallisé d'un certain nombre d'espèces chimiques et minéraless chromium is added the colour is blue, forming the true logiques," Comples rendus, vol. xlvi. p. 764; A. Verneuil, “ Pro Oriental sapphire. The real colouring matter of the Oriental duction artificielle des rubis par fusion," 'ibid. vol. cxxxv. P. sapphire is not definitely known, some chemists considering it to J. Boyer, La Synthèse des pierres précieuses (Paris, 1909).* .*** be chromium and others cobalt. Artificial sapphires have been GEMBLOUX, a town in the province of Namur and on the made of a fair size and perfectly transparent by the addition borders of Brabant, Belgium, 25 m. S.E. of Brussels on the main of cobalt to the igneous bath of alumina, but the writer does line to Namur and Luxemburg. Pop. (1904) 4643. It is a busy not consider them equal in colour to true Oriental sapphire. place with large railway and engine works, and the junction for
The Oriental Emerald.-The stone known as emerald consists several branch lines. On the 31st of January 1578 Don John chemically of silica, alumina and glucina. Like the ruby, it owes of Austria gained here a signal victory over the army of the its colour to chromium, but in a different state of oxidation. As provinces led by Antony de Goignies. already mentioned, there is another stone which consists of GEMINI (“The Twins," i.e. Castor and Pollux), in astronomy, crystallized alumina coloured with chromium, but holding the the third sign in the zodiac, denoted by the symbol II. It is chromium in a different state of oxidation. This is called the also a constellation, mentioned by Eudoxus (4th century B.c.) Oriental emerald, and, owing to its beauty of colour, its hardness and Aratus (3rd century B.C.), and catalogued by Ptolemy, 25 and rarity, it is more highly prized than the emerald itself and stars, Tycho Brahe 25, and Hevelius 38. By the Egyptians this commands higher prices. The Oriental emerald has been constellation was symbolized as a couple of young kids; the produced artificially in the same way as the ruby, by adding a Greeks altered this symbol to two children, variously said to be larger amount of chromium to the alumina bath and regulating Castor and Pollux, Hercules and Apollo, or Triptolemus and the temperature.
Iasion; the Arabians used the symbol of a pair of peacocks. The Oriental Amethyst.—The amethyst is rock crystal (quartz) Interesting objects in this constellation are: a Geminorum or of a bluish-violet colour. It is one of the least valuable of the Castor, a very fine double star of magnitudes 2.0 and 2.8, the precious stones. The sapphire, however, is found occasionally of fainter component is a spectroscopic binary; Geminorum, a a beautiful violet colour; it is then called the Oriental amethyst, long period (231 days) variable, the extreme range in magnitude and, on account of its beauty and rarity, is of great value. It is being 3.2 to 4; 5 Geminorum, a short period variable, 10-15 days, evident that is to the igneous bath of alumina some colouring the extreme range in magnitude being 3.7 to 4.5; Nora matter, such as manganese, is added capable of communicating Geminorum, a star discovered in 1903 by H. H. Turner a violet colour to the crystals of alumina, the Oriental amethyst of Oxford; and the star cluster M.35 Geminorum, a fine and will be the result. Oriental amethyst has been so formed artifici- bright, but loose, cluster, with very little central condensation. ally, but the stone being known only as a curiosity to mineralogists GEMINIANI, FRANCESCO (c. 1680-1762), Italian violinist, and experts in precious stones, and the public not being able to was born at Lucca about 1680. He received lessons in music discriminate bei ween the violet sapphire and amethystine quartz, from Alessandro Scarlatti, and studied the violin under Lunati there is no demand for the artificial stone.
(Gobbo) and afterwards under Corelli. In 1714 he arrived in The Oriental Topas.—The topaz is what is called a semi- London, where he was taken under the special protection of the precious stone. It occurs of many colours, from dear white to carl of Essex, and made a living by teaching and writing music. pink, orange, yellow and pale green. The usual colour is from In 1915 he played his violin concertos with Handel at the English straw-yellow to sherry colour. The exact composition of the court. After visiting Paris and residing there for some time, colouring matter is not known; it is not entirely of mineral he returned to England in 1755. In 1761 he went to Dublin, origin, as it changes colour and sometimes fades altogether on. where a servant robbed him of a musical manuscript on which exposure to light. Chemically the topaz consists of alumina, he had bestowed much time and labour. His vexation at this silica and fluorine. It is not so hard as the sapphire. There is loss is said to have hastened his death on the 17th of September also a yellow variety of quartz, which is sometimes called "false 1762. He appears to have been a first-rate violinist, but most topaz." The Oriental topaz, on the other hand, is a precious of his compositions are dry and deficient in melody. His Ari stone of great value. It consists of clear crystalline sapphire of Playing the Violin is a good work of its kind, but his Guida