CHALCEDONY and agate, and their endless varieties, are composed mainly of silica, but the separate individual crystals are so small as to be invisible to the unaided eyesight, and occasion-ally are so extremely minute that the structure is almost amorphous. The colour and appearance vary greatly, depending upon the impurities contained in the stone, and, since both have been made a criterion for differentiation of types, a host of names have come into use, none of which are susceptible of strict definition. On the whole, these stones may be divided into two groups : chalcedony, in which the structure is concretionary and the colour comparatively uniform, and agate, in which the arrangement takes the form of bands, varying greatly in tint and colour.
The refraction, though double in the individual, is irregular over the stone as a whole, and the indices approximate to V550. The specific gravity ranges from 2.62 to 2.64, depending upon the impurities present. The degree of hardness is about the same as that of quartz, namely, 7 on Mohs’s scale. All kinds are more or less porous, and stones of a dull colour are therefore artificially tinted after being worked.
The term chalcedony, derived from the name of a town in Asia Minor, is usually confined to stones of a greyish tinge. Stones artifically coloured an emerald green have been cut and put upon the market as ` emeraldine.’ Carnelian is a clear red chalcedony, and sard is somewhat similar, but brownish in tint. Chrysoprase is apple-green in colour, nickel oxide being supposed to be the agent. Prase (cf. p. 240), which is a dull leek-green in hue, may also in part be referred here ; the name comes from , a leek. Plasma, which may have the same derivation, is a brighter leek-green. Jasper is a chalcedony coloured blood-red by iron oxide, while bloodstone is a green chalcedony spotted with jasper ; they are popular stones for signet rings. Flint, an opaque chalcedony, breaks with a sharp cutting edge, and was much in request with early man as a tool or a weapon ; its property of giving sparks when struck with steel rendered it invaluable before the invention of matches. Hornstone is somewhat similar, but more brittle, while chert is a flinty rock.
Agate, named after the river Achates in Sicily, where it was found at the time of Theophrastus, has a peculiar banded structure, the bands being usually irregular in shape, following the configuration of the cavity in which it was formed. Moss-agate, or mocha-stone, contains moss-like inclusions of some fibrous mineral. Onyx is an agate with regular bands, the layers having sharply different colours ; when black and white, it has, in days gone by, been employed for cameos. Sardonyx is similar in structure, but red and white in colour. Agate is used in delicate balances for supporting the steel knife-edges of the balance itself and of the panholders, and is largely employedespecially when artificially colouredfor umbrella handles and similar articles.
Chalcedony and agate are found the whole world over, but India, and particularly Brazil, are noted for their fine carnelians and agates.