GEM

GEM, in Natural History, a common name for all precious stones; of which there are two classes, the pellucid and semipellucid.

The bodies composing the class of pellucid gems are bright, elegant, and beautiful fossils, which are found in small detached masses, extremely hard, and of great lustre.

The bodies composing the class of semipellucid gems, are stones naturally compound, not inflammable or soluble in water, found in detached masses, and composed of crystalline matter debased by earth: however, they are but slightly debased; and are of great beauty and brightness, of a moderate degree of transparency, and are usually found in small masses.

The knowledge of gems depends principally on observing their hardness and colour. Their hardness is commonly

Gem. commonly allowed to stand in the following order: The diamond the hardest of all; then the ruby, sapphire, jacinth, emerald, amethyst, garnet, carnel, chalcedony, onyx, jasper, agate, porphyry, and marble. This difference, however, is not regular and constant, but frequently varies. Good crystals may be allowed to succeed the onyx; but the whole family of metallic glassy fluors seems to be still softer.—In point of colour, the diamond is valued for its transparency, the ruby for its purple, the sapphire for its blue, the emerald for its green, the jacinth for its orange, the amethyst carnel for its carnation, the onyx for its tawny, the jasper, agate, and porphyry, for their vermilion, green, and variegated colours, and the garnet for its transparent blood red.

All these gems are sometimes found coloured and spotted, and sometimes quite limpid and colourless. In this case the diamond cutter or polisher knows how to distinguish their different species by their different degrees of hardness upon the mill. For the cutting or polishing of gems, the fine powder of the fragments of those that are next in degree of hardness is always required to grind away the softer; but as none of them are harder than the diamond, this can only be polished by its own powder.

Cronstedt observes of gems in general, that the colour of the ruby and emerald are said to remain in the fire, while that of the topaz flies off: hence it is usual to burn the topaz, and thence substitute it for the diamond. "Their colours (says our author) are commonly supposed to depend upon metallic vapours; but may they not more justly be supposed to arise from a phlogiston united with a metallic or some other earth? because we find that metallic earths which are perfectly well calcined give no colour to any glass: and that the manganese, on the other hand, gives more colour than can be ascribed to the small quantity of metal which is to be extracted from it." M. Magellan is of opinion, that their colour is owing chiefly to the mixture of iron which enters their composition; but approves the sentiment of Cronstedt, that phlogiston has a share in their production, it being well known that the calces of iron when dephlogisticated produce the red and yellow colours of marble, and when phlogisticated to a certain degree produce the blue or green colours.

With regard to the texture of gems, M. Magellan observes, that all of them are foliated or laminated, and of various degrees of hardness. Whenever the edges of these laminae are sensible to the eye, they have a fibrous appearance, and reflect various shades of colour, which change successively according to their angular position to the eye. These are called by the French chatoyantes; and what is a blemish in their transparency, often enhances their value on account of their scarcity. But when the substance of a gem is composed of a broken texture, consisting of various sets of laminae differently inclined to each other, it emits at the same time various irradiations of different colours, which succeed one another according to their angle of position. This kind of gems has obtained the name of opals, and are valued in proportion to the brilliancy, beauty, and variety of their colours. Their crystallization, no doubt, depends on the same cause which produces that of salts, earths, and metals, which is treated of under the article CRYSTALLIZATION. The

following table shows the component parts of gems according to the analysis of Bergman and M. Achard; the letter B prefixed to each denoting Bergman's analysis, and A that of Achard.

But later analyses shew that the component parts are different from the above, particularly the colouring matters which are here ascribed to iron. See MINERALOGY.

The chrysoprase from Koseinitz in Silesia was likewise analyzed by M. Achard; who found that it contained 456 grains of siliceous earth, 13 of calcareous, six of magnesia, three of copper, and two of iron. "This (says M. Magellan) seems to be the only gem that contains no argillaceous earth."

Imitation or Counterfeiting of Gems in Glass. The art of imitating gems in glass is too considerable to be passed without notice: some of the leading compositions therein we shall mention upon the authority of Neri and others.

These gems are made of pastes; and are noway inferior to the native stones, when carefully made and well polished, in brightness or transparency, but want their hardness.

The general rules to be observed in making the pastes are these: 1. That all the vessels in which they are made be firmly luted, and the lute left to dry before they are put into the fire. 2. That such vessels be chosen for the work as will bear the fire well. 3. That the powders be prepared on a porphyry stone; not in a metal mortar, which would communicate a tinge to them. 4. That the just proportion in the quantity of the several ingredients be nicely observed. 5. That the materials be all well mixed; and, if not sufficiently baked the first time, to be committed to the fire again, without breaking the pot; for if this be not observed, they will be full of blisters and air bladders. 6. That a small vacuity be always left at the top of the pot, to give room to the swelling of the ingredients.

To make paste of extreme hardness, and capable of all the colours of the gems, with great lustre and beauty.—Take of prepared crystal, ten pounds; salt of poverine, six pounds; sulphur of lead, two pounds: mix all these well together into a fine powder; make the whole with common water into a hard paste; and make this paste into small cakes of about three ounces weight each, with a hole made in their middle; dry them in the sun, and afterwards calcine them in the firiest part of a potter's furnace. After this, pow-

der them, and levigate them to a perfect fineness on a porphyry stone, and set this powder in pots in a glass furnace to purify for three days: then cast the whole into water, and afterwards return it into the furnace, where let it stand 15 days, in which time all foulness and blisters will disappear, and the paste will greatly resemble the natural jewels. To give this the colour of the emerald, add to it brass thrice calcined; for a sea green, brass simply calcined to a redness; for a sapphire, add zaffer, with manganese; and for a topaz, manganese and tartar. All the gems are thus imitated in this, by the same way of working as the making of coloured glasses; and this is so hard, that they very much approach the natural gems.

The colour of all the counterfeit gems made of the several pastes, may be made deeper or lighter according to the work for which the stones are designed; and it is a necessary general rule, that small stones for rings, &c. require a deeper colour, and large ones a paler. Besides the colours made from manganese, verdigris, and zaffer, which are the ingredients commonly used, there are other very fine ones which care and skill may prepare. Very fine red may be made from gold, and one not much inferior to that from iron; a very fine green from brass or copper; a sky colour from silver, and a much finer one from the granates of Bohemia.

A very singular and excellent way of making the paste to imitate the coloured gems is this: Take a quantity of saccharum saturni, or sugar of lead, made with vinegar in the common way; set it in sand, in a glass body well luted from the neck downwards; leave the mouth of the glass open, and continue the fire 24 hours; then take out the salt, and if it be not red but yellowish, powder it fine, and return it into the vessel, and keep it in the sand heat 24 hours more, till it becomes as red as cinnabar. The fire must not be made so strong as to melt it, for then all the process is spoiled. Pour distilled vinegar on this calcined salt, and separate the solution from the dregs; let the decanted liquor stand six days in an earthen vessel, to give time for the finer sediment to subside; filter this liquor, and evaporate it in a glass body, and there will remain a most pure salt of lead; dry this well, then dissolve it in fair water; let the solution stand six days in a glazed pan; let it subside, then filter the clear solution, and evaporate it to a yet more pure white and sweet salt; repeat this operation three times; put the now perfectly pure salt into a glass vessel, set it in a sand heat for several days, and it will be calcined to a fine impalpable powder of a lively red. This is called the sulphur of lead.

Take all the ingredients as in the common composition of the pastes of the several colours, only instead of red lead, use this powder; and the produce will well reward the trouble of the operation, as experience has often proved.

A paste proper for receiving colours may be readily made by well pounding and mixing six pounds of white sand cleansed, three pounds of red lead, two pounds of purified pearl-ashes, and one pound of nitre. A softer paste may be made in the same manner, of six pounds of white sand cleansed; red lead, and purified pearl-ashes, of each three pounds; one pound of nitre, half a pound of borax, and three ounces of arsenic. For

common use a pound of common salt may be substituted for the borax. This glass will be very soft, and will not bear much wear if employed for rings, buckles, or such imitations of stones as are exposed to much rubbing; but for ear-rings, ornaments worn on the breast, and those little used, it may last a considerable time.

In order to give paste different colours, the process is as follows: For

Amethyst. Take ten pounds of either of the compositions described under Colouring of Glass, one ounce and a half of manganese, and one drachm of zaffer; powder and fuse them together.

Black. Take ten pounds of either of the compositions just referred to, one ounce of zaffer, six drachms of manganese, and five drachms of iron, highly calcined; and proceed as before.

Blue. Take of the same composition, ten pounds; of zaffer, six drachms; and of manganese, two drachms; and proceed as with the foregoing.

Chrysolite. Take of either of the compositions for paste above described, prepared without saltpetre, ten pounds, and of calcined iron five drachms; and pursue the same process as with the rest.

Red Cornelian. Take of the compositions mentioned under Colouring of Glass, two pounds; of glass of antimony, one pound; of the calcined vitriol called scarlet ochre, two ounces; and of manganese, one drachm. Fuse the glass of antimony and manganese with the composition; then powder them, and mix them with the other, by grinding them together, and fuse them with a gentle heat.

White Cornelian. Take of the composition just referred to, two pounds; of yellow ochre well washed, two drachms; and of calcined bones, one ounce. Mix them, and fuse them with a gentle heat.

Diamond. Take of the white sand, six pounds; of red lead, four pounds; of pearl-ashes, purified, three pounds; of nitre two pounds; of arsenic five ounces; and of manganese, one scruple. Powder and fuse them.

Eagle-marine. Take ten pounds of the composition under GLASS; three ounces of copper highly calcined with sulphur; and one scruple of zaffer. Proceed as before.

Emerald. Take of the same composition with the last nine pounds; three ounces of copper precipitated from aqua fortis; and two drachms of precipitated iron. See EMERALD, MINERALOGY Index.

Garnet. Take two pounds of the composition under GLASS; two pounds of the glass of antimony, and two drachms of manganese. For vinegar garnet, take of the composition for paste, described in this article, two pounds; one pound of glass of antimony, and half an ounce of iron, highly calcined: mix the iron with the uncoloured paste, and fuse them: then add the glass of antimony powdered, and continue them in the heat till the whole is incorporated.

Gold or full Yellow. Take of the composition for paste ten pounds; and one ounce and a half of iron strongly calcined; proceeding as with the others.

Deep Purple. Take of either of the compositions for paste, ten pounds; of manganese, one ounce; and of zaffer, half an ounce.

Ruby. Take one pound of either of the compositions

Gem. tions for paste, and two drachms precipitate of gold by tin; powder the paste, and grind the calx of gold with it in a glass, flint, or agate mortar, and then fuse them together. A cheaper ruby paste may be made with half a pound of either of the above compositions, half a pound of glass of antimony, and one drachm and a half of the calx of gold; proceeding as before.

Sapphire. Take of the composition for paste, ten pounds; of zaffer, three drachms and one scruple; and of the calx Cassi, one drachm. Powder and fuse them. Or the same may be done, by mixing with the paste one-eighth of its weight of smalt.

Topaz. Take of the compositions under GLASS ten pounds, omitting the saltpetre; and an equal quantity of the Gold-coloured hard GLASS. Powder and fuse them. See TOPAZ, MINERALOGY INDEX.

Turquoise. Take of the composition for blue paste already described, ten pounds; of calcined bone, horn, or ivory, half a pound. Powder and fuse them.

Opaque white. Take of the composition for paste ten pounds; and one pound of calcined horn, ivory, or bone; and proceed as before.

Semitransparent white, like opal. See OPAL, MINERALOGY INDEX.

To the above we shall add the following receipts and processes, contained in a memoir by M. Fontanieu of the Royal Academy of Sciences at Paris, and said to have met with much approbation.

I. Of the Bases. Although the different calces of lead are all adapted to produce the same effect in vitrification; yet M. Fontanieu prefers lead in scales, and next to that minium, as being the most constantly pure. It is necessary to sift through a silk sieve the preparations of lead one wishes to make use of in the vitrification, in order to separate the grosser parts, as also the lead found in a metallic state when white lead in scales is employed.

The base of facitious gems is calx of lead and rock crystal, or any other stone vitrifiable by the calces already mentioned. Pure sand, flint, and the transparent pebbles of rivers, are substances equally fit to make glass; but as it is first necessary to break the masses of crystal, stones, or pebbles, into smaller parts; so by this operation particles of iron or copper are frequently introduced, and to these dust or greasy matters are also apt to adhere. Our author therefore begins by putting the pounded crystal or pebbles into a crucible, which he places in a degree of heat capable of making the mass red hot; he then pours it into a wooden bowl filled with very clear water; and shaking the bowl from time to time, the small portions of coals furnished by the extraneous bodies swim on the surface of the water, and the vitrifiable earth, with the iron, &c. rests on the bottom. He then decants the water; and having dried the mass, he pounds it, and sifts the powder through the finest silk sieve: he then digests the powder during four or five hours with marine acid, shaking the mixture every hour. After having decanted the marine acid from the vitrifiable earth, he washes the latter until the water no longer reddens the tincture of turnsol. The said earth being dried, is passed through a silk sieve, and is then fit for use. Nitre, salt of tartar, and borax, are the three species of salts that enter with quartz and the several calces of lead into M. Fontanieu's vitrifications.

VOL. IX. Part II.

Gem. Much of the success in the art of making coloured stones depends on the accurate proportion of the substances made use of to form the crystal which serves as a base to the facitious stones. After having tried a great variety of receipts, our author found they might be reduced to the following.

1. Take two parts and a half of lead in scales, one part and a half of rock crystal or prepared flints, half a part of nitre, as much borax, and a quarter part of glass of arsenic. These being well pulverized and mixed together, are to be put into a Hessian crucible, and submitted to the fire. When the mixture is well melted, pour it into cold water: then melt it again a second and a third time; taking care, after each melting, to throw it into fresh cold water, and to separate from it the lead that may be revived. The same crucible should not be used a second time, because the glass of lead is apt to penetrate it in such a manner as to run the risk of losing the contents. One must also be careful to cover the crucible well, to prevent any coals getting into it, which would reduce the calx of lead, and spoil the composition.

2. Take two parts and a half of white ceruse, one part of prepared flints, half a part of salt of tartar, and a quarter part of calcined borax: melt the mixture in a Hessian crucible, and then pour it into cold water; it is then to be melted again, and washed a second and a third time, the same precautions being observed as for the first base.

3. Take two parts minium, one part rock crystal, half a part of nitre, and as much salt of tartar: this mixture being melted, must be treated as the former.

4. Take three parts of calcined borax, one part of prepared rock crystal, and one part of salt of tartar; these being well mixed and melted together, must be poured into warm water: the water being decanted and the mass dried, an equal quantity of minium must be added to it; it is then to be melted and washed several times as directed above.

5. That called by our author the Mayence base, and which he considers as one of the finest crystalline compositions hitherto known, is thus composed: Take three parts of fixed alkali of tartar, one part of rock crystal or flint pulverized: the mixture to be well baked together, and then left to cool. It is afterwards poured into a crucible of hot water to dissolve the frit; the solution of the frit is then received into a stone-ware pan, and aqua fortis added gradually to the solution till it no longer effervesces: this water being decanted, the frit must be washed in warm water till it has no longer any taste: the frit is then dried, and mixed with one part and a half of fine ceruse or white lead in scales; and this mixture must be well levigated with a little distilled water. To one part and a half of this powder dried add an ounce of calcined borax: let the whole be well mixed in a marble mortar, then melted and poured into cold water as the other bases already described. These fusions and lotions having been repeated, and the mixture dried and powdered, a 12th part of nitre must be added to it, and then melted for the last time; when a very fine crystal will be found in the crucible.

6. As a composition for furnishing very fine white stones: Take eight ounces of ceruse, three ounces of

Gen. rock crystal pulverized, two ounces of borax finely powdered, and half a grain of manganese: having melted and washed this mixture in the manner directed above, it will produce a very fine white crystal.