DIAMONDS.—Mr. Tenneat read a paper on the Koh-i-noor diamond. He considered the great,Indian diamond, the Russian diamond, and Koh-i-hoor, were separate portions of the original Koh-i-noor procured from the mines of Golconda. That opinion was supported by the peculiar relation of the cleavage planes to the other sides, which could not otherwise be accounted for. A very interesting discussion ensued, in which Professor Tennent described the progress already made in the grinding and polishing of what he called the English portion of this far-iamed stone. Referring to the diamonds procured in the Brazils, he related a fact which, he said, was toldto him by a gentleman from Brazil. A slave in tfiat country was one day wading in a river in search of the precious gems to be found imbedded in the strand, when he struck his crow bar in a spot which surprised 1iirn "by its hollow sound. He repeated its blows, and soon struck the iron through a crust of siliceous particles cemented together by oxide of iron. On removing the concrete mass, the slave discovered a bed of diamonds, which were afterwards disposed of lor 300,000. Such an immense number of diamonds being thrown upon the market, so overstocked it that nearly all the dealers became bankrupt, and upon the diamonds being introduced into England, the glut was so great that the results to the trade were equally disastrous, only three English houses being able to stand up against it. One of those persons was a gentleman in Leadenhall street, who was so largely engaged in the trade, that he had actually shown him (Mr. Tennent) a peck lull of diamonds. Sir David Brewster entered into some account of the same diamond. He said—In the course of last spring, I was requested by H. R. H. Prince Albert to give my opinion respecting different forms into which it was proposed to reduce the Koh-i-noor diamond, in order to make it an ornamental gem. In the state it then was, it exhibited an inferior display of colors to its glass model, and it was only by surrounding it by a number of vivid lights that its colored refractions could be developed. Having had occasion to observe some remarkable phenomena in small portions of diamond, an account of which was published in the Transactions of the Geological Society for 1836,1 was desirous of examining so large a mass of diamond as the Koh-i-noor, before it was reduced in size, and covered with facets which would not permit it to be examined. His Royal Highnessreadily granted my request, and I had thus an opportunity of submitting it to the scrutiny of polarised light. In place of producing no action upon this species of light, as might have been expected from its octahedral structure, it exhibited streaks of polarised tints, generally parallel to one another, but, in some places, of an irregular lorm, and riling to the yellow of the first order of colors. These tints and portions oi polarised light were exactly the same as those which I had long ago found in manj other diamonds, and published in the Edin burgh Transactions for 1815 and 1816. In placing the Koh-i-noor under a microscope of considerable power, I observed in it, and also in each of the two small diamonds which accompanied it, several minute and irregular cavities surrounded with sections of polarized light, which could only have been produced by the extensive action of a compressed gas, or fluid that had existed in the cavities when the diamond was in a soft state. In an external cavity, shown in the model, and which had been used for fixing the gold setting, I observed with common light a portion of yellow light, indicating a yellow substance. Mr. Garrard and others considered it as gold rubbed off the gold setting; but as gold is never yellow by transmitted light, I considered the color as produced by a yellow solid substance of unknown origin. Sir Henry de la Beche having suggested to me that it would be desirable to make a general examination of the principal diamonds in London, I went next day to the British Museum, and found there an excellent specimen, which threw some light on the yellow solid to which I have referred. This specimen was a piece of colorless diamond, uncut, and without any crystalline facets, about three or four-tenths of an inch broad, and about the twelfth of an inch thick, and on its surface there lay a crystal of yellow diamond, with the four planes of semi-octohedron. This singular fact was illustrated by a large model placed beside it. Upon examining the original I noticed a pretty large cavity in the thickness of the specimen, with tke extremity of which the yellow octohe-dron was connected, and finding a portion of amorphous yellow diamond in the other end of the cavity, I had no doubt that the yellow crystal had emerged, in a fluid state, from the cavity when it was accidentally opened, and had immediately crystallized on the surface of cleavage, f am well aware that such an opinion makes a good demand upon the faith of the mineralogist, but to those who have seen as I have done, the contents of fluid cavities, in crystal, solidifying, and even crystallizing in the face of the cleavage, while another portion "of the contents ot the cavity escaped in gas—to those who have seen in the topaz cavities numbers of regularly formed crystals, jome of which, after being fused by heat, instantly re-crystalize, the conclusion I have drawn will be stripped of its apparent extravagance. In examining a number of diamonds in the Museum of the East India Company, to which Col. Sykes kindly obtained me access, and about forty or fifty in the possession of Messrs. Hunt and Roskell, I found many containing large and irregular cavities of the most fantastic shapes, and all of them surrounded with irregular patches of polarized light, of high tints, produced, undoubtedly, by a pressure from within the cavities, and modified by their form. Among these specimens I found one or two black diamonds, not black from a dark coloring matter like that in smoky quartz, but black from the immense number of cavities which it contained. Tavernier has described a large and curious diamond which throws some light on the subject of this notice. It,contained, in its very centre, a large black cavity. The diamond merchants refused to purchase it. At last a Dutchman bought it, and by cutting it in two, obtained two very fine diamonds. The black cavity through which he cut, was found to contain eight or nine carats of what Tavernier calls black vegetable mud. [This is a subject which we know will greatly interest Prof. Horsford of Cambridge.