Diamond, a genus of siliceous earths, and the hardest of all the stones hitherto discovered ; it is in general transparent, but is sometimes found of a rose-colour, or inclining to green, blue, yellow, or black.

The most valuable diamonds are those of a complexion similar to that of a drop of water: their price also increases in proportion to the regularity of their form, and accordingly as they are free from spots, stains, flaws, specks, and cross veins. Diamonds are found chiefly in India and South America, whence they are brought to Europe in a rough state, in the form of roundish pebbles with shining surfaces. There is, however, a kind of diamonds, which are but little esteemed, found in various parts of Europe, and also in this country, in the county of Cornwall, where they are called Cornish diamonds. These may, with more propriety, be termed crystals ; they are found in digging the tin-mines of Cornwall, and are, in general, bright and clear, except towards the root, where they are coarse, and assume a whitish colour.

It is remarkable that genuine diamonds, when exposed to the rays of the sun, attract light which they again emit, and appear luminous, in the dark. The largest jewel of this description, in the world, is at present in the royal treasury of Portugal: it is of an oyal figure, measures about 4 inches by 3, weighs 1680 carats, or 121/2| ounces, and is valued at 224 millions sterling.

Independently of the purposes to which the diamond is subservient as an ornament, especially in the dress of females, the smaller particles of it have, since the 16th century, been employed for cutting glass; and, when reduced to an impalpable powder, are very useful for polishing other precious stones, as well as for engraving on those which possess an inferior degree of hardness.

For the valuation of diamonds of all weights, Mr. D. Jefferies, an ingenious jeweller, ,who published a treatise on diamonds and pearls, several years since, lays down the following rule ! He first supposes the value of a rough diamond to be settled at 21. per carat, at a medium ; then, to find the value of diamonds of greater weight, he directs to multiply the square of their weight by 2, and the product is the value required. On this principle, Mr. Jefferies has constructed tables of the price of diamonds from 1 to 1000 carats, which the curious reader will find in the work before mentioned, of which a new edition appeared a few years since, in 8vo. price 12s.