GEM, a common appellation for aM precious stones, and particularly for those which are employed in jewellery. Brilliancy of lustre, richness of hue, and the most perfect transparency, are the desiderata of this class of minerals; and the value of those specimens in which such characters are combined is enhanced according to their size, in an extremely rapid ratio. At the head of the gems stands the diamond, which, for brilliancy of lustre, or water as it is termed, has not a rival. As elsewhere stated, it is principally the production of certain parts of Hindustan and Brazil, where it occurs in alluvial soil, or in conglomerate of the most recent formation. The oriental ruby, when perfect in transparency and colour, and of considerable size, vies with the diamond in value. Its most noted locality is the Capellan Mountains, near Syrian, in the kingdom of Ava. Next in order is the sapphire, which varies from a rich dark blue to a very slight tinge of the same colour, sometimes presents distinct colours in the same specimen, and is frequently transparent and colourless. It is met with in considerably larger masses than the preceding, and is only found in Ceylon. The peculiar colour of the emerald is well known. No gem is more frequently made mention of in sacred history, and none even at the present day stands in higher estimation amongst the crowned heads of the East. The aquamarine [7:10:379]of lapidaries is, mineralogically speaking, a pale-blue variety of the same species, though one of very inferior value. Topazes occur under an infinity of forms, and are found in most quarters of the globe. From the changes which they undergo when exposed to heat, besides the fine colours they present naturally, topazes are peculiarly adapted to the purposes of the lapidary. The richest coloured garnets are the production of Ceylon and Greenland. Chrysolites of a brilliant pale-green colour are brought to Great Britain from Constantinople; hyacinths of a deep red from Ceylon; and tourmalines of a great variety of hues from all parts of the world. The opal, and many varieties of quartz, as the amethyst, onyx, sarde, cats-eye, and agate, are also occasionally included under the general denomination of gem.

Generally speaking, the different species of precious stones occur in small masses. All of them are found in a crystalline state, and present the peculiar forms which nature has assigned them. Many of these forms are singularly beautiful; and their variety, particularly in the diamond and topaz species, is as great as any other class of the mineral kingdom affords. It is not, however, under these splendid forms that the gems are most commonly found. Most of them are the productions of India, Ceylon, Pegu, or Brazil, where, having been washed from their matrix in some of the primitive ranges, they are collected, in a rolled and rubbed state, in the channels of the rivers. Every trace of their original form is thus frequently lost; and, to add to the distress of the crystallographer, fine stones rarely come through the {lands of Indian lapidaries without receiving farther abrasion or cutting. This circumstance of their so rarely presenting distinctive forms renders the peculiar hardness of this class of stones a most important characteristic, and one which, whatever be their external shape or appearance, can never be mistaken. By this means too the real stone is easily distinguished from the fictitious, which is occasionally made to bear so close a similitude, that an unpractised eye finds it almost impossible to detect the difference.