Onyx, Sardonyx, etc. 165
Onyxes,
agates, etc., are without difficulty stained to any colour by
artificial means ; for black, the stone is first boiled in honey, oil,
or sugared water, and then in sulphuric acid, which carbonizes the oil
or sugar the stone has absorbed ; for red, protosulphate of iron is
added, thereby leaving the iron in the form of an oxide ; for the deep
blue colour sometimes seen on onxyes, yellow prussiate of potash is
added to the protosulphate of iron. It must be remarked that it is
only the more porous parts of the stone which absorb the sugar, and the
carbonization heightens greatly the contrast between the white and
black layers.
The
stones styled nicolo or onicolo are a variety of onyx, having a deep
brown ground, overlaid by a layer of bluish-white ; the white layer
being excessively thin, the dark colour shows through. These stones are
principally found in Bohemia and the Tyrol. Amongst the ancients they
were much valued for engraving cameos and intaglios. The name nicolo is
derived from the Greek Νικόλαος.
The
sardonyx is of a reddish-brown colour, consisting of alternate layers
of chalcedony and carnelion. The name is derived from the union of the
sard and the onyx, which were formerly considered to be different
stones. The sard is supposed to have derived its name from the ancient
Sardis, or from the Greek word σαρξ, flesh, on account of its
colour, which somewhat resembles raw flesh. Both onyx and sardonyx are
much used in jewellery, especially for signet rings ; the different
layers
