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 ?a???d?? 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, Chalcedony and agate are found the whole world over, but India, and particularly Brazil, are noted for their fine carnelians and agates. |