CRYSTAL, a species of stones of the quartz kind, belonging to the siliceous class. It always appears, when 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 semitransparent, and white or of a milk colour. 2. Opaque and red, or of a cornelian 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, and are of various colours; or they lie in a clayey earth, and have both pyramids, but no prism. They are found at Blackenburg upon the Hartz, and at Morferosh in the Silverland in Transylvania.
The coloured transparent crystals derive their tinge from an exceedingly small portion of metallic calces, 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 Valay 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 flints. The same author reckons among crystals the oriental topaz, the hyacinth, the oriental sapphire, and the amethyst. Mr Daubenton has always looked upon this last as a quartz or 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 Bomaré, 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 Eibach in the Wallais; 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 pyramidal 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 accidental sapphire.
The orders of pure crystal are three: The first is No 95.
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 aqua fortis, and effervescence 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 sparg 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 Archard 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 filtering 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 Archard 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. In the natural way many of the more compound fossil bodies are formed chiefly either of crystal, or of spar, a body in many things resembling it. The original formation and coalescence of those bodies of which spar is the basis, we know, may have been but of yesterday, since we have evident proofs that spar is concretizing to this day, and that sparry bodies are forming every moment. This is evident from the sparry stalactite in the arches of modern buildings, particularly in one lately built as the new bridge at Westminster; the roofs of the arches of which were filled with these spars within a year after they were built. It is also demonstrable that the spars are not formed of matter exuding from the stone, since brick arches abound equally with them; and the brick vault which supports part of the grand terrace at London, was some time ago so full of them that there was not room to walk. These observations sufficiently demonstrate the growth of spar; but the vegetation of crystal remained dubious till Dr Hill showed by some experiments that crystal, as well as spar, is dissolved in every kind of water, even such as appears to be most pure and clear. This is also probable from an observation of Neumann's, who tells us, that he has seen leaves, stalks of plants, hay, straw, hogs bristles, &c. inclosed in sprigs of crystal. From the regular forms in which these natural crystals are found, the regular arrangement of salts into different figures takes the name of crystallization, and both are probably owing to the same cause*. Henckel gives us a remarkable account of the formation of crystal out of human urine. He once filled a large round glass-vessel half way up with the recent urine of a young lad, and tying a bladder over the mouth of the vessel, set it in a stove for four years together, never stirring it during that whole time. At the end of this time he found a number of small white stones growing in the inside of the glass; they were of the size of an oat-seed, of a prismatic figure, and tolerably pellucid: they stuck so fast to the sides of the glass that they could not be washed off by the shaking about of the urine; and when taken out had no saline taste, and were not soluble even in hot water.
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 aqua fortis, 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 mere rubbing with a cloth.
Natural crystal may be reduced by calcination into
VOL. V. PART II.
a slate proper for making glass with alkaline salts, and thus becomes a very valuable fritt. 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.