CORUNDUM. Adamantine spar.—,Korund and De-mantspath, Werner ; Telesie, Corindon, Haüy.

This mineral was long known in Europe under the name of adamantine spar. It was imported from India, where it was used in cutting and polishing gems; but its history was wholly unknown until the late Right Honourable Charles Greville, to whom the science of mineralogy is so deeply indebted, obtained a clue by which he arrived at one locality at least of this interesting mineral.

In the year 1767, Mr Berry, a very eminent seal-engraver in Edinburgh, received from Dr Anderson of Madras a box of crystals of adamantine spar, with information that it was with this substance all stones except diamond were cut and polished in India. Mr Greville obtained from his friend Colonel Catheart some crystals, which that gentleman had also got from Dr Anderson twenty years afterwards, and ι to which the native name of corundum was attached. By this means Mr Greville was enabled to put the exertions of others in requisition, and after various fruitless attempts, through the assistance of Mr Edward Garrow, then (1792) residing at Trichinopoly, he at length obtained a minute account of the stone, as known in that part of Hindustan. It seems that the searching for and working with this substance is consigned, like most other branches of trade, to one particular caste; hence all others are wholly ignorant of it, and those who belong to it are communicative or otherwise, just as they may consider their interest affected. By dint of perseverance he at last discovered that the corundum was found in the neighbourhood of Condrastra, a village about fourteen miles from Permetty, which, according to Rennell’s map, is situated on the south side of the Cavery, not far from Caranel. Hither Mr Garrow repaired; and being, as he says, the first European whom the natives had seen, was as great a curiosity among them as Landers proved to the Ashantees. Condrastra he found to be a very small hamlet, occupied by five families, whose race had obtained a prescriptive right as miners to work in the pits. The implements by which their operations were carried on were of a very rude nature; a broad pointed heavy iron crow with a wooden handle is the only one mentioned. With this they formed their excavation, in a soil composed of gritty granite, masses of which, several pounds weight, on being brought to the surface, were crushed, and the crystals collected.

The receipt of this collection afforded Mr Greville the means of writing the interesting account of the corundum stone, which we find in the Transactions of the Royal Society of London, read 7th June 1798, in which its identity with the sapphire, previously pointed out by Pelletier and De Lametherie in 1787, was finally confirmed. It is singular, that during that year other philosophers were occupied with the investigation of this mineral, namely, Werner in Germany, and Rome de Lisle in France; the former having sent to the latter two wooden models, which he pronounced to be the shape of the oriental ruby, and similar in form to the crystal of adamantine stone dissected by Pelletier and his colaborateur, and confirming the hint originally thrown out by Rome de Lisle, of a connection between the adamantine stone and the gems distinguished by the appellation of oriental.

Thus the name of corundum was given to the genus, and was generally adopted by mineralogists.

Bournon, in his paper on this stone, read before the Royal Society in 1802, divides it simply into perfect and imperfect; and had he added a third sub-species, under the name of granular or compact, his genus would have been sufficiently comprehensive.

Mohs has described this mineral as the third species of his genus corundum, under the name of rhombohedral corundum, which comprehends the sapphire and ruby, with the other different-coloured telesies of Haüy, included in Bournon’s perfect corundum. The brown and greenish-grey coloured varieties, and the emery stone from Madras, Naxos, &c. are also comprehended under the same head by Mohs.

The first of these, the perfect corundum, presents an infinite variety of colours. Red and blue are the most common; and the former, when perfect in transparency and colour, and of considerable size, vies with the diamond itself in value; but it is seldom known to exceed half an inch in length, with a proportionate breadth. The very perfect crystals of dark blue are next in value, and then the yellow; but the resemblance of the last to topaz when cut depreciates it considerably 7 . These varieties are principally the produce of Pegu and Ceylon, but are found in several other localities in Europe, especially in Auvergne and St Gothard, but in small or foul stones.

Of the imperfect corundum we have abundance of the greenish-grey colour from the Mysore and the Malabar coast, whilst the Carnatic and China afford the brown-coloured variety in rude imperfect crystals. In the Mysore it was originally found among the vegetable soil, evidently derived from the decomposition of the granite on which the soil rests, as noticed in the account by Mr Garrow.

The corundum occurs always imbedded in its matrix, not in drusy cavities. It is very hard, and difficultly frangible; but the natural joints are so perfect in the specimens of the greenish-grey coloured variety, that it is very easily cleavable into the primitive rhomb. In the granular variety from Naxos the sapphire occurs in distinct concretions of considerable size, and of a good colour. The compact variety is used by the natives of India for grinding and polishing all the different-coloured gems, which is done by friction in the simplest manner. Use soon indents grooves on the surface of the stone; and it is from these grooves that the polished stones from India obtain such a constantly similar form, convex, oval, or circular. Pounded corundum, moistened with a little water, expedites the operation of grinding and polishing, and is also used in cutting and slicing stones. A bit of twisted wire stretched on a bow of bamboo of about three feet long, forms, with the grinding-stone, the whole necessary implements of an Indian lapidary. At Gellivara, in Sweden, it occurs in greenish-grey particles, dispersed in iron ore. For other particulars, see Mineralogy. ∣