GEMS AND PRECIOUS STONES, A LECTURE By PROFESSOR JAMES TENNANT, F.G.S.

Daily Southern Cross, Volume XIV, Issue 1019, 3 April 1857, Page 3

GEMS AND PRECIOUS STONES, A LECTURE By PROFESSOR JAMES TENNANT, F.G.S.

(Concluded from our issue of the 27th ult.) Mr. Maiden, who is an extensive importer, has subsequently informed me, that "the mines of Suruu and Sincora, in the province of Hah'ta, also produce diamonds. The washings of the former place have been abandoned on account of the smallness of the productian and badness of the quality of the stones, the greater part of which appear to me to be crystallized, and to have the appearance of having been exposed to the action of fire. The Sincora mine was discovered about the year 1843 by a mulatto miner, who had previously been engaged at the washings of Surua. On retiring from Surua, he proceeded into the interior alone and with but fourteen days' provision, in search of other washings, which he succeeded in finding in a few days. After labouring with success for some days, he found that hit provisions were barely sufficient to carry him back to his home. He was therefore obliged to relinquish his labours, and return with the stones he had collected, which he offered for 6ale to some of the parties who had been engaged at the Surua mine. As the stones were of a different quality and shape to any they had seen before, they taxed him with having discoveied a new mine. For pome time he strongly denied having done so, but on being thrown into pribon, and accused of having stolen the diamonds, he confessed his discovery, and, on promise of making it known, was released. Six or eight months after, from ten to fifteen thousand persons had collected on the spot. The produc- j tion was so abundant, that, for the first two years, it is supposed thatnearly 600,000 carats were extracted, and forwarded to Europe. In consequence, the prices of rough diamonds fell from 38s. and 40s. to 18s. and 205., and, some eight months after, went down as low as 12s. and 14s. The decrease in value, increased difficulties of finding, and high prices of provisions, together with the unhealthinesß of the climate, caused many of the miners to retire ; in consequence of which the production has considerably diminished, and mayJLaJ^ this piesent time, 1852, be, on an average, about 130;000 carats per arm. The quality of the Sincora diamonds is inferior to those found in Minas Geraes or Cuyaba. Since Dr. Cliffe's article was written, a new mine has been discovered, m the province of Minas Geraes, on the margins of the river Patrocinho. It is denominated Bagagem, ami has produced some large stones." A stone was found there in 1851, weighing 117 ear ats. Having dilated so largely upon the diamond, time will permit me to give only a brief notice of the other precious stones. I will first describe Coruudum. It consists almost entirely of pure alumina, and is mxt in hardness to the diamond, from which it differs in ciystalline form, occuiring commonly in six-sided prisms, sometimes terminated at one or both emls by six-sided pyramids. In some respects, these crystals resemble those of quartz, but may be distinguished therefrom by being more elongated ; they are also of a supeiior specific gravity and haidness. The presence of certain metallic oxides in minute quantities imparts colour to this substance. When of a pink colour, it is called Oriental ruby; when violet, Oriental amethyst; when yellow, Oriental topaz ; when blue, it is called a sapphire. Diverging rays of light are also exhibited by this stone, whence it is called Astena, or S'arstone. Some very fine specimens, exhibited by Messrs. Ilunt and Roskell, formed a portion of Mr. Hope's collection. The Oriental ruby, when of fine colour and free from imperfections, is more valuable than a diamond of equal weight, and a larger price is given tor itiu India. The finer varieties of corundum, as used in jewellery, are chiefly brought from Ceylon. The Chrytoberyl, or Cymophane, is next in haidness to corundum : it is not much worn, although a beautiful stone, frequently possessing a milky or opalescent appearance; when cut en cabochon it exhibits a floating white band of light, from which the name cymophane is derived. Some choice examples gave visitors an opportunity of becoming acquainted with i's appeal a.nee. I next allude to the Spinel. This is found crystallized. Its red varieties are occasionally mistaken fur the oriental ruby, but may be distinguished by a less degree of hardness and specific gravity. The Great Exhibition contained some remaikabie specimens of Topaz of different colours, consisting of white, yellow, pink, and blue. It is found in rhombic crystals, often deeply striated in the longitudinal direction of the prism. White topazes are obtained from New South Wales, Ceylon, and Brazil. The majority from Brazil are of a yellow or pink colour : the latter may be produced artificially by the application of heat ; to the yellow variety. There were some large topazes, sent as pebbles from the -Cairngorm mountains in Scotland ; and a clergyman from thdt district informs me that he has often picked up such, supposing them to be mere quartz peblles. These mountains are famous for a coloured variety of transparent quartz, which bears their name; but the distinction may always be made, even should the crystalline form be destroyed, by either breaking it or taking the specific gravity. In the quartz, the fracture will be cur\ed like a piece of broken glass ; in the topaz it will be perfectly smooth, and at right angles to the axis of the original prism; pieces may be readily detached parallel to this flat surface. An incident occurred to myself in the Exhibition, which shows the utility and safety of resorting to, as an important test, the specific gravity. Many of my hearers doubtless remember a beautiful stone in the Russian department, which was labelled Phenakite. This stone was of a bluish colour, and very biilliant. In company with several other persons, I examined it closely Opinions were various concernmg its nature ; one f»aul it was a beryl, another thought it was an aquamarine, and a third pronounced it a topaz. The owner gave permission to scratch the suiface ; but at my suggestion it was decided, as the safer test, to ascertain its specific gravity. I was allowed to take the stone to the Royal Institution, and weigh it in their scales. Here is the stone with which I compared it, namely, a topaz, in the natural state. On weighing both, I discovered the result to be precisely the same, therefore the so-called phenakite was proved to be a topas. The process of taking the specific gravity of a stone is extremely simple. In the case before us, we first weighed the stone in the air, and found it weighed 562 grains ; we then immersed it in water, and found it to , weigh only 404 grains. Thus it had lost 108 grains, j and by dividing the first weight by the difference, we , obtained the specific gravity. It is a simple sum.which any schoolboy can do, and yet it is one that is seldom resorted to by persons who spend thousands of pounds in thepujchase of gems. By dividing 562 by 158, we obtained 3-5 as the specific gravity of this stone, whereas had it been phenakite, the specific gravity would have been 27 . The Great Exhibition, doubtless, contained the finest known Emerald, and through the kindne&s of its owner, we have an opportunity of renewing our acquaintance with it this evening. Comparing it with those magnificent emeralds on the portion of the trappings of the horse in the Indian Collection, I find it to be half an inch longer than the largest of them. The profusion and beauty of the emeralds and other precious stones in that department were almost bewildering, yet not one could be found to rival the extraordinary stone before us. It has been variously described by several authors. This beautiful stone was placed in my case at the Exhibition, and was its greatest ornamait, as well as a sliikins; feature in the general collection. It is 2 inches in length, and measures across the three diameters 2~ inch., 2 onefifth inch., and 1£ inch, and weighs 8 oz, 18 dwts. It is ! a six-sided prism, and has a small piece of quartz adhering, which if detached would probably weigh 3 or 4 dwts. This emerald was obtained from Muro, near Santa Fe de Bogota, in Granada. Several other fine specimens from the same locality were placed near it. I believe this fine crystal was brought to England by Don Pedro, from whom it was purchased by the Duke of Devonshire. The beautiful colour of the emerald is due to lor 2 percent, of oxide of chromium The emera'd consists of silica 68, alumina 15, glucina, with a trace of lime, oxide of iron, and chromium. This stona break-* readily at right angles to its axis, and the Eastern lapidaries have availed themselves of this quality. In the girdle of an Indian chief, which formed a remarkable object in the Indian collection, most of the emerald" wereslices of the natural crystal, polished and mounted. The natural faces of the crystal were, in several instances, preserved, and the slices thus obtained were surrounded with diamonds, some in the natural state. This was the usual mode of mounting them previous to 1456. The slices of emerald in this girdle are a quarter of an inch thick. Besides this emerald girdle, there were some curious emerald ornaments, containing three large stones in each ; also an emerald and diamond turban ornament, a diamond and emerald bridle, ore. These are a portion of the jewels previously described by the Hon. Miss Eden. . The Beryl is a substance which differs but little from the emerald, except in colour ; it has the same crystalline form, hardness, and specific gravity. The Australian department contained some good specimens of this stone and there is no doubt that when the mineral treasures of Australia come to be better known, stones fit for the finest jewellery may be obtained from it. Interesting specimens were exhibited from the Mourne Mountains, county Down, and Cairngorm, in Aberdeenshire. A gigantic opaque beryl from North America, unfit for jewellery, weighing 80 lbs., was in the Exhibition^ but there is one in the British Museum of about the same size, and of a more perfect form. Pebbles of quartz

are frequently mistaken for beryl* , anil vice versa When these substances are crystallized, the moans ot distinguishing them are very simple ; quartz is striated transversely on the planes of the prism ; beryl is striated longitudinally ; and by sacrificing one or two crystals, and observing the fracture, the truth may be ascertained If emerald or beryl, the fracture will be in planes, like the "lieoo in thp Indian gi'dle, or the elp«vnge "t the topaz : if quartz, the fracture will nevei be in d | straight line, but oonchoidal Mr. A. Hupp exhibited a , fine transparent beryl, forming the hilt of a sworu. , ! Shortly before the close of the Exhibition, a precious opal in its matrix, from Hungary, was shown m the ; Austrian department. Thi-- stone was remarkable lor its varied colours, and attiacted much notice. Several ( interesting specimens were also shown in the American department. Many of the jewellers made a display of precious garnets, or carbuncles, in the various collections of precious stones. The crystalline form of garnet is that ot a rhombic dodecahedron, and the same having all the edges replaced by six-sided planes. Tourmaline.' Only a few of these were shown. Ihis mineral is more valuable for optical than ornamental purposes. Many beautiful examples of artificial gems were exhibited, and it required an experienced eye to distinguish them from the real specimens. They may, however, in all cases, be readily distinguished by their inferior hardness, yielding readily to the file, or to a wystnl of quartz. . .. In conclusion, I desire to draw the attention of all persons likely to travel ir foreign countiies, especially to Australia, the Cape of Good Hope, India, and Canada, to the great impoitance of a knowledge of minerals. There is no country in the world possessing a wider range of territory or greater mineral wealth than Great Britain ; consequently, there is none in which the study ot mineralogy is more important; yet the ignorance which prevails on th c subject is astonishing. I am frequently receiving packages and letters of enquiry from our colonies, containing pebbles of quartz and bits of shining iron pyrites, which a few simple experiments would render unnecessary. A very small amount of knowledge as to the method of testing minerals, especially as to their specific gravity, would save months of anxious suspense, which must occur while waiting a reply from England. But lam not without hope that the ignorance which leads to such mistakes as these will gradually be dispelled, and that the nation will yet be effectually roused to a sense of. the great importance of Natural History in every scheme that mar henceforth be devised for the public or private tuition of our people.