Gems, Facti′′tious

These, with few exceptions, are made of very pure, fusible, highly transparent, and dense gla , usually termed ‘ PASTE ’ or ‘ STRASS ,’ which is generally formed of oxide of lead, pota a, and silica, with small quantities of other ingredients to increase the brilliancy and clearne . The characteristic tints are imparted by the addition of metallic oxides. The beauty of artificial stones and gems depends, chiefly, upon the tint of the real stones being exactly imitated, and upon proper care and skill being exercised in the cutting, polishing, and mounting them. All the coloured gla es, and enamels, may be worked up into artificial gems. Table of the Hardne , Specific Gravity, and Refractive Power of the principal Gems and Precious Stones , and some other Minerals ; compiled expre ly for this work . | Name. | Relative Hardne . | Specific Gravity. | Index of Reflection. | | Agate | 12 | 2·6 | | | Amethyst (occidental) | 11 | 2·7 | | Calcareous spar | 6 | 2·7 | | Chalk | 3 | 2·7 | | Chrysolite | 10 | 3·7 | | Cornelian | 11 | 2·7 | | Crystal | 11 | 2·6 | | Diamond (bluish) | 19 | 3·3 | 2·439 | | Diamond (cubic) | 18 | 3·2 | | Diamond (from Ormus) | 20 | 3·7 | | Diamond (pink) | 19 | 3·4 | | Diamond (yellowish) | 19 | 3·3 | | Diamond (average colourle ) | 19 to 20 | 3·3 to 3·55 | | Emerald | 12 | 2·8 | | | Fluor spar | 7 | 3·5 | 1·434 | | Garnet | 12 | 4·4 | 1·815 | | Gla | various | 2·3 to 3·62 | 1·525 to 2·028 | | Gla (crystal or flint) | 3·0 to 3·6 | 1·830 to 2·028 | | Gla (plate) | 2·5 to 2·6 | 1·514 to 1·542 | | Gypsum | 5 | 2·3 | | | Jasper (green) | 11 | 2·7 | | Jasper (reddish yellow) | 9 | 2·6 | | Onyx | 12 | 2·6 | | Opal | 10 | 2·6 | | Quartz | 10 | 2·7 | 1·548 | | Ruby | 17 | 4·2 | 1·779 | | Ruby (pale, from Brazil) | 17 | 3·5 | | Ruby (spinelle) | 13 | 3·4 | 1·764 | | Sapphire (deep blue) | 16 | 3·8 | 1·794 | | Sapphire (paler) | 17 | 3·8 | | Sardonyx | 12 | 2·6 | | | Schoerl | 10 | 3·6 | | Topaz | 15 | 4·2 | | Topaz (Bohemian) | 11 | 2·8 | | Topaz (whitish) | 14 | 3·5 | | Tourmaline | 10 | 3·0 | | Zeolite | 8 | 2·1 | | Zircon | — | — | 1·961 | MM. Fremy and Feil have lately succeeded in manufacturing artificial corundum, ruby and topaz, having a composition the same as the natural stones. The proce by which they have effected this consists in fusing together at a red heat, in the furnace of a gla works for a considerable time, a fusible aluminate (such as aluminate of lead), and some silicious body. The silica is found to unite with the lead, and to liberate the alumina in the crystalline form. When equal weights of alumina and red lead are heated together in a crucible made of some refractory silicious substance, the above conditions if the temperature has been maintained sufficiently long and high ensue, and there is found in the crucible at the end of the operation a layer of silicate of lead, and very frequently another of pure crystallised alumina or corundum. The ruby colour is given by adding to the mixture in the crucible two or addition of a small quantity of oxide of cobalt, with a trace only of bichromate of potash. A film of silicate of lead very frequently adheres to the ruby crystals, and this has to be removed. In some instances, however, the crystals occur nearly pure, and are precisely similar to the natural gems in crystalline form, composition, hardne , and lustre. Upon being heated, the artificial ruby, like the natural one, loses its rose colour, and recovers it again on cooling. It is said that the factitious gems hitherto obtained are not, as a rule, equal in lustre to the natural ones, and are consequently not so well suited for jewellers’ work; also that they do not present to the lapidary conditions favorable to cleavage or cutting. They are, however, very well adapted for the works of watches. See Enamels , Pastes , .