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Encyclopedia Britannica - Main :: GAG-GEO |
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GEM (Lat. gemma, a bud,from the root gen, meaning " to produce,"or precious stone; in the latter sense the Greek term is > d>os) , 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-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: (I) Mineralogy
I. MINERALOGY
The gem-stones form a small conventional group of minerals, including principally the diamond, ruby, sapphire, emerald and opal. Other stones of less valuesuch as topaz, spinel, chrysoberyl, chrysolite, zircon and tourmalineare sometimes called " fancy stones." Many minerals still less prized, yet often used as ornamental stones,like moonstone
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. A high degree of hardness is an essential property of a gem-stone, for however beautiful and brilliant a mineral
withstand the abrasion to which articles of personal decoration are necessarily subjected. Even if not definitely scratched, the polished stone becomes dull by wear. Imitations in paste may be extremely brilliant, but being comparatively soft they soon lose lustre when rubbed. In the article MINERALOGY 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 gem-stones (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 gem-stones are harder than quartz, though precious opal, turquoise
moonstone
sharp
drawn
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 stonefrom another. Moreover, it is a character very easily determined and can be applied to cut stones without injury. The relative weightiness of a stone is called its specific gravity, and is often abbreviated as S.G. The number given in speclfk gravity. the description of a mineral
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 3i 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 stone be true topaz or false topaz (quartz), it is merely necessary 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 a gem. 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 Crystal~me fully under CRYSTALLOGRAPHY. A few substances cleavage. used as ornamental stoneslike opal, turquoise
obsidian and amberare amorphous or without crystalline form; whilst others, like the various stones of the chalcedony-group, 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
Many crystallized gem-stones exhibit cleavage, or a tendency to split in definite directions. The lapidary recognizes a " grain " in the stone. When the cleavage is perfect, as in topaz, it may render the working of the stone difficult, and produce incipient cracks in the cut gem. Flaws due to the cleavage planes are called " feathers." The octahedral cleavage of the diamond is taken advantage of in dressing the stone before cutting it. The cutting of gem-stones is explained under LAPIDARY. 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 sub-stances 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 has not been detected in the sapphire. Probably the most common absorption. It is sometimes useful to examine the behaviour of a stone under the action of the Rontgen rays. 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 Wlsmkh .re' i 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 reachirg 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. 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 chryso- beryl may be regarded as aluminates; turquoise and chemical beryllonite are phosphates; and a great number of aPOs 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
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 superstiformerly held in esteem not only for their beauty and Woos, rarity but for the medicinal and magical powers with which they were reputed to be endowed. Up to comparatively recent
mineral pigments are compounds of iron, manganese, copper and 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
The colours of some gem-stones are greatly affected by radio-activity, 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 remark-able change of this character, see ALEXANDRITE. As the optical properties of minerals are fully explained under End of Article: GEM (Lat. gemma, a bud,from the root gen, meaning " to produce,"or precious stone; in the latter sense the Greek term is > d>os) If you wish, you can link directly to this article.
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