in natural history, a genus of the semipellucid gems: they are of an even and regular, not tabulated structure; of a semi-opaque crystal-line basis; and variegated with different colours, but those ever disposed in form of mists or clouds, and, if nicely examined, found to be owing to an admixture of various coloured earths, but imperfectly blended in the mass, and often visible in distinct molecular.—It has been doubted by some whether the ancients were at all acquainted with the stone we call chalcedony; they having described a Chalcedonian carbuncle and emerald, neither of which can at all agree with the characters of our stone; but we are to consider that they have also described a Chalcedonian jasper which seems to have been the very same stone, as they describe by the word turbida, which extremely well agrees with our chalcedony.

There are four known species of the chalcedony.

1. A bluish white one. This is the most common of all, and is found in the shape of our flints and pebbles, in masses of two or three inches or more in diameter: it is of a whitish colour, with a faint cloud of blue diffused all over it, but always in the greatest degree near the surface. This is a little less hard than the oriental onyx. The oriental chalcedonies are the only ones of any value; they are found in vast abundance on the shores of rivers in all parts of the East Indies, and frequently come over among the ballast of the East-India ships. They are common in Silesia and Bohemia, and other parts of Europe also; but with us are less hard, more opaque, and of very little value.

2. The dull milky-veined chalcedony. This is a stone of little value; and is sometimes met with among our lapidaries, who mistake it for a kind of nephritic stone. It is of a somewhat yellowish white or cream colour, with a few milk-white veins. This is principally found in New-Spain.

3. The third is a brownish, black, dull, and cloudy one, known to the ancients by the name of smoky jasper, or jaspis capitis. This is the least beautiful stone of all the clasps; it is of a pale brownish white, clouded all over with a blackish mist, as the common chalcedony is with a blue. It is common both in the East and West Indies, and in Germany; but is very little valued, and is seldom worked into anything better than the handles of knives.

4. The yellow and red chalcedony is greatly superior to all the rest in beauty; and is in great repute in Italy, though very little known among us. It is naturally composed of an admixture of red and yellow only, on a clouded crystalline basis; but is sometimes found blended with the matter of common chalcedony, and then is mixed with blue. It is all over of the milky hue of the common chalcedony. This is found only in the East-Indies, and there not plentifully. The Italians make it into beads, and call these chalcedonies; but they are not determinate in the use of the word, but call beads of several of the agates by the same name.—All the chalcedonies readily give fire with steel, and make no effervescence with aquafortis.