CHALCEDONY, AGATE, ETC.
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 occasionally 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 1·550. 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 χαλκηδών the name of a town in
Asia Minor, is usually confined to stones of a greyish tinge. Stones
artificially 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 πράσμον, 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 employed—especially when
artificially coloured—for 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.