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 Chemical composition. 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 faced and backed by a slice of genuine stone. By the action of 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.
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 σμάραγδος 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 composition and in all physical characters, save colour.
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 Superstitions. 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 as amulets. The Orphic poem Λιθικά, reputed to be of very early though unknown date, is rich in allusions to the virtues of many of the gem-stones. Many of the medical and other virtues of precious stones were evidently attributed to them on the well-known doctrine of signatures. Thus, the blood-red colour of a fine jasper suggested that the stone would be useful in haemorrhage; a green jasper would bring fertility to the soil; and the purple wine-colour of amethyst pointed to its value as a preventive 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 Gem-Cutter’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 (σφραγίς, sigillum, intaglio), or mainly for artistic effect (imagines ectypae, 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 lathewise, 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 (δακτυλοκοιλογύφος; see Athenische Mitteilungen des Arch. Inst. xv. p. 333). Unfortunately the relief is incomplete, and the published illustration inadequate. It would seem, however, that a revolving tool was supported by a kind of mandrel, and actuated in primitive fashion by a bow. An alternative plan of working was to use a splinter of diamond set in a handle and applied like a graver. Both systems are clearly indicated by Pliny, who in one passage (H.N. xxxvii. 60) states that diamond splinters are sought out by gem engravers and set in iron, and so easily hollow out stones of 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.