Volume 2

Ure's dictionary of arts, manufactures, and mines : containing a clear exposition of their principles and practice / by Robert Hunt. ; assisted by F.W. Rudler.

Ure, Andrew, 1778-1857.

Date:: 1878-1979

Credit: Ure's dictionary of arts, manufactures, and mines : containing a clear exposition of their principles and practice / by Robert Hunt. ; assisted by F.W. Rudler. Source: Wellcome Collection.

40/1060 (page 24)

$Diamonds possessod of no extraordinary magnitude, but of a good form and a pure -water, may be valued by a certain standard rule. In a brilliant, or rose diamond ot regular proportions, so much is cut away that the weight of the polished gem does not exceed one-half the weight of the diamond in the rough state ; whence the value of a cut diamond is esteemed equal to that of a simihir rough diamond of double weight exclusive of the cost of workmanship. The weight and value of diamonds are reckoned by carats of 4 grains each ; and the comparative value of two diamonds of equal quality, but different weights, is as the squares of these weights respectively. The aver- age price of rough diamonds that are worth working, is about 21. for one of a single carat; but as a polished diamond of one carat must have taken one of two carats, its price in the rough state is double the square of 21., or 8^. Therefore to estimate the value of a wrought diamond, ascertain its weight in carats, double that weight, and multiply the square of this product by 21. Hence, a wrought diamond of 1 carat is worth £8 7 carats is worth £392 2 „ 32 8 „ 612 3 ,. 72 9 „ 612 4 128 10 „ 800 5 „ 200 20 „ 3,200; 6 „ 288 beyond -which weight the price can no longer rise in this geometrical progression, from the small number of purchasers of such expensive toys. A very trifling spot or flaw of any kind lowers exceedingly the commercial value of a diamond. The preceding rule was given by Jefferies many years ago, and though no doubt correct enough in its day, is of little or no value now. The folio-wing formula has been recently given by Schrauf, of Vienna {Edelsteinkunde, 1869): Let a be the weight of a given stone in carats, and b the value of a diamond of one carat; then the value of the stone of a carats is approximately ? (a + 2) h. After all, it may be doubted whether any really useful rule can be given to connect weight -with value; certainly there is none applicable to large stones. The market value is often capricious ; at one time, stones of a particular size will be fashionable, and -will therefore fetch unusually high prices. Moreover, much depends on the quality or' water' of the gem; if the stone be ' off-colom'ed,' or if it contain cloudy im- perfections known in the trade as ' milk' or ' salt,' its value is very greatly diminished. On the other hand, brilliantly-coloured diamonds are prized as ' fancy stones,' and are eagerly purchased at high prices. Diamonds are used not only as decorative gems, but for more useful purposes, as for cutting glass by the glazier, and all kinds of hard stones by the lapidary. On the structure of the glaziers' diamond we possess some very interesting obser- vations and reflections by Dr. WoUaston. He remarks, that the hardest substances brought to a sharp point scratch glass, indeed, but do not cut it, and that diamonds alone possess that property ; which he ascribes to the pcciiliarity of its crystallisation in rounded faces, and curvilinear edges. For glass-cutting, those rough diamonds are always selected which are sharply crystallised, hence called diamond sparks ; but cut diamonds are never used. The inclination to be given to a set diamond in cutting glass is comprised within very narrow limits; and it ought, moreover, to be moved in the direction of one of its angles. The curvilinear edge adjoining the curved faces, entering as a wedge into the furrow opened up by itself, thus tends to separate the parts of the glass ; and in order that the crack which causes the separation of the vitreous particles may take place, the diamond must be held almost perpendicular to the surface of the glass. The Doctor proved this theory by an experiment. If, by suitable cutting with the wheel, we make the edges of a spinel, ruby, or corundum- telesie (sapphire), curvilinear, and the adjacent faces curved, these stones will cut glass AS well as a glaziers' diamond, but being less hard than it, they -will not preserve this property so long. He found that upon giving the surface of even a fragment of flint the same shape as that of the cutting diamond, it acquired the same property; but, from its relative softness, was of little duration. The depth to which the fissure caused by the glaziers' diamond penetrates does not seem to exceed the two-hundredth of an inch. The following remarks by Mr. Tennant cannot fail to bo of interest, and, as pointing out the errors which have been frequently committed through ignorance, are of great value:— ' By attending to the forms of tlie crystal, we are quite sure that we shall not find the emerald, sapphire, zircon, or topaz in the form of a cube, octahedron, tetrahedron, or rhombic dodecahedron; nor the diamond, spinel, or garnet in that of a six-sided$