CRYSTAL, a species of stone belonging to the quartz or siliceous genus. It always appears, where there has been no interruption to its crystallization, in hexagonal prisms pointed at both ends. It is found of different kinds and colours. 1. Opaque or semi-transparent, and white or of a milk colour. 2. Opaque and red, or of a carnelian colour, from Oran in Barbary. 3. Opaque and black, from the same place. 4. Clear. The specific gravity of these kinds of crystals is from 2650 to 2700. Professor Bergman extracted from them about six parts of argilla and one of calcareous earth per hundred weight; but Mr Gerhard found some so pure as to contain neither. 5. Clear and blackish brown; the smoky topaz, or rauch topaz of the Germans. It is found at Egan in Norway, and at Lovisa in Finland. These crystals are said to become clear by boiling them in tallow. 6. Clear and yellow; found in Bohemia, and sold instead of topazes. 7. Clear and violet-coloured; the amethyst from Saxony, Bohemia, and Dannemore in Upland. The most transparent of these are called false diamonds, Bristol, Kerry stones, Alençon diamonds, &c. 8. Colourless rock crystal, properly so called, found in Bohemia, the province of Jemtland, and many other places. 9. Pyramidal crystal with one or two points. These have no prismatic shape, but either stand upon a base in cavities of quartz veins, have only a single pyramid,
Crystal. pyramid, and are of various colours; or they lie in clayey earths, and have both pyramids, but no prism. They are found at Blackenburg upon the Hartz, and at Mosserosh in the Silverland in Transylvania.
The coloured transparent crystals derive their tinge from an exceedingly small portion of metallic oxide, but lose them entirely when strongly heated. They are called false gems; viz. the red from Oran in Barbary, false rubies; the yellow from Saxony, false topazes; the green from Dauphiny, very rare, false emeralds or prases; the violet from Vil in Catalonia, false amethysts; the blue from Puy in Valais in France, false sapphires. There are likewise opal or rainbow crystals, the various colours of which are thrown out in zones across the surface. They make a very fine appearance, though they never shine like the oriental opal.
M. Fourcroy makes a remarkable difference between the crystals and quartz, by affirming that the former are unalterable in the fire, in which they neither lose their hardness, transparency nor colour; while the quartz loses the same qualities, and is reduced by it to a white and opaque earth. He classes the rock crystals,
I. According to their form, viz. 1. Insulated hexagonal crystals ending in pyramids of six faces, which have a double refraction, or show two images of the same object when looked through. 2. Hexagonal crystals united, having one or two points. 3. Tetrahedral, dodecaedral, flattened crystals; and which, though hexagonal, have nevertheless their planes irregular. 4. Crystals in large masses, from the island of Madagascar, which have a simple refraction.
II. With regard to their colour, as being either diaphanous, reddish, smoky, or blackish.
III. With regard to accidental changes, some are hollow; some contain water within one or more cavities: some are cased one within the other; some are of a round form, as the pebbles of the Rhine; some have a crust of metallic calces or of a pyrites; some are found crystallized in the inside of a cavity; while some seem to contain amianthus or asbestos; and others contain shirls. The same author reckons among crystals the oriental topaz, the hyacinth, the oriental sapphire, and the amethyst. M. Daubenton has always looked upon this last as a quartz of a crystal.
When the rock crystals are semitransparent or intermixed with opaque veins, they are called by the Swedish lapidaries milk-crystals. When they are found in the form of round pebbles, which is occasioned by their being tossed about and rubbed against one another by floods or by the sea, they are called by the English lapidaries pebble-crystals. They come from the Indies, Siberia, and other places.
According to Bomare, the rock-crystals are generally formed upon or among quartz, which shows their great affinity, and are to be found in all parts of the world. The greatest quantity of them is brought from Mount Saint Gothard in Switzerland. Large pieces of these, weighing from 5 to 800 pounds, were found there at Grimselberg; another of about 1200 pounds weight was found some years ago at Fishbach in the Valais: and a piece six feet long, four wide, and equally thick, was found in the island of Madagascar, where these natural productions are of the most extraordinary size and perfection.
In the imperial collection at Vienna, there is a pyra-
mid crystal vase two ells in height, cut wholly out of one piece. It is usual with the largest crystals of the German mountains to be full of cracks and flaws, and to be so constructed internally as to show all the prismatic colours; but the above-mentioned ones were quite free from these blemishes, and resembled columns of the purest glass, only much clearer than any glass can be made. Crystal is also found in many parts of Britain and Ireland. About Bristol it is found of an amethystine tinge. In Silesia and Bohemia in Germany it is found stained with the colours of the ruby, sapphire, emerald, and topaz; in which case jewellers take great advantage of it, selling it under the name of occidental sapphire.
The orders of pure crystal are three; the first is perfect columnar crystals, with double pyramids, composed of 18 planes, in an hexangular column, terminated by an hexangular pyramid at each end: the second order is that of perfect crystals, with double pyramids, without a column, composed either of 12 or of 16 planes, in two hexangular pyramids, joined closely base to base, without the intervention of any column; the third order is that of imperfect crystals, with single pyramids, composed either of 12 or 10 planes, in an hexangular or pentangular column, affixed irregularly at one end to some solid body, and terminated at the other by an hexangular or pentangular pyramid.
These are all the general forms into which crystal, when pure, is found concreted; but under these there are almost infinite varieties in the number of angles, and the length, thickness, and other accidents of the columns and pyramids.
When crystal is blended with metalline particles at the time of its formation, it assumes a variety of figures wholly different from these, constituting a fourth order, under the name of metalline crystals; when that metal is lead, the crystal assumes the form of a cube; when it is tin, of a quadrilateral pyramid, with a broad base; when iron, the crystal is found concreted in rhomboidal figures: these crystals are very common about mines; but the common spars, which are liable to be influenced in the same manner by the metals, and to appear in the very same form, are to be carefully distinguished from them. There is one very easy test for this purpose, which is, that all spars are subject to be dissolved by aquafortis, and effervesce violently only on its touching them: but it has no such effects on crystal.
The pebble-crystal is common enough in all parts of the world; but that which is formed of hexangular columns, affixed to a solid base at one end, and terminated by a hexangular column at the other, is infinitely more so: this is what we call spig or rock crystal, and is the species described by most authors under the name of crystal of the shops, or that kept for medicinal uses.
With regard to the formation of crystals, it is certain that they must have been once in a soft state, since some are found to have water in their cavities. Professor Bergman obtained 13 regular formed crystals, by suffering the powder of quartz to remain in a vessel with fluor acid for two years. These were about the size of small peas, and were less hard than quartz. Mr Magellan informs us, that he received from Mr Achard
Crystal. Achard two crystals, one of the sparry kind, and the other as hard and transparent as rock crystal. The first he procured by means of calcareous earth, and the latter from the earth of alum, both dissolved in water impregnated with fixed air, the water filtrating very slowly through a porous bottom of baked clay. The apparatus is described by the author in the Journal de Physique for January 1778: but though the process was attempted by Mr Magellan, and afterwards a second time by Mr Achard himself, neither of them were able to succeed. Mr Morveau, however, in the first volume of the Dijon Memoirs for 1785, asserts that he has produced a very small artificial crystal; and gives the proper method for succeeding in the process.
Crystal is frequently cut; and lustres, vases, and toys, are made of it as of other beautiful stones. For this purpose it is to be chosen perfectly clear and transparent. It is to be tried by aquafortis, or by drawing it along a pane of glass. The genuine crystal will not be affected by the acid, and will cut glass almost like a diamond. When any piece of workmanship of natural crystal is become foul and dark, the following method is to be used for recovering its brightness without hurting the polish. Mix together six parts of common water and one part of brandy; boil these over a brisk fire, and let the crystal be kept in it, in a boiling state, a quarter of an hour; then take it out and rub it carefully over with a brush dipped in the same liquor; after this it is to be wiped with a napkin, and by that means its surface will be perfectly cleaned, and rendered as bright as at first, without any injury to the points of the cutting or the polish of the planes or faces, which would probably have happened had the cleaning been attempted by more rubbing with a cloth.
Natural crystal may be reduced by calcination into a state proper for making glass with alkaline salts, and thus becomes a very valuable frit. The method of doing it is as follows: calcine natural crystal in a crucible; when it is red hot, throw it into cold water. Repeat this eight times, covering the crucible, that no dust or ashes may get in among the crystal. Dry this calcined mass, and reduce it to an impalpable powder.
Colouring Crystal, for the imitation of gems. See DOUBLET.