Chalcedony

Chalcedony (often misspelled Calcedony), a beautiful mineral of the quartz family, consisting of quartz with some admixture of opal. It derives its name from Chalcedon in Bithynia, near which it is found in considerable abundance, and has been known by the same name from ancient times. It never occurs in crystals, but usually in mammillary, botryoidal, or stalactitic forms, lining or entirely filling the cavities of rocks, and more particularly old igneous rocks, such as the basalt-rocks of Scotland, the Farœ Isles, Iceland, &c. It constitutes the whole or the principal part of many agates. It is generally translucent, sometimes semi-transparent, has a somewhat waxy lustre, and is in colour generally white or bluish white, sometimes reddish white, sometimes milk white, less frequently gray, blue, green, yellow, brown, or even black. Its fracture is even, or very slightly conchoidal. Chalcedony is much used in jewelry, for brooches, necklaces, and ornaments of all sorts, the largest pieces being sometimes made into little boxes, cups, &c. It was much used by the ancients, and many beautiful engraved specimens appear in antiquarian collections. Chalcedonies with disseminated spots of brown and red were once very highly prized, and were called Stigmites or St Stephen's stones. Petrified plants are sometimes found in chalcedony, in which they appear to have been incased whilst it was in course of formation. Specimens of chal- cedony are sometimes found inclosing a little water in the interior, which gives them a very beautiful appearance; but the water easily escapes, and to prevent this, rings or other ornaments made of such stones are kept in distilled water when not worn. The ancients set a very high value on these enhydrites (Gr. en, 'in,' and hydor, 'water'). See JASPER.