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ONYX, a banded chalcedony or striped agate, composed of white layers alternating with others of black, brown or red colour. A typical onyx consists of two or more black and white strata, whilst the term sardonyx is applied to the stone if it contains red or brown bands (see Sardonyx). Probably those varieties which show red and white zones originally suggested the name “onyx,” from Gr. ὄνυξ (a finger nail), since the colours of such stones may be not unlike those of the nail. The onyx when worked by the lapidary was often designated by the diminutive ὀνύχιον; and at the present day the term nicolo, a corruption of the Italian diminutive onycolo, is applied to an onyx which presents a thin layer of chalcedony deriving a bluish tint from the subjacent black ground. The Hebrew soham is translated in the authorized version of the Old Testament “onyx,” but the revised version gives in some of the passages an alternative marginal reading of “beryl.” The position of the land of Havilah, which yielded the onyx-stone, is uncertain.

India has for ages supplied the finest onyxes, and hence jewellers apply the expression “Oriental onyx” to any stone remarkable for beauty of colour and regularity of stratification, quite regardless of its locality. As far back as the 1st century the author of the Periplus Maris Erythraei mentions the onyx among the products of Plythanae, a locality probably identified with Paithan on the Godavari; and he further states that the stones were taken down to Barygaza, the modern Broach, where the agate trade still flourishes. It is probable that the early Greeks and Romans derived their prized agate-cups from this locality. The Indian onyx is found, with agate and jasper-pebbles, in river gravels derived from the disintegration of the amygdaloidal volcanic rocks of the Deccan. A great deal of onyx now sold is obtained from South American agates, cut in Germany. It often happens that the lower deposits in an agate-nodule are in horizontal layers, forming onyx, while the other deposits have adapted themselves to the curved contours of the cavity. The onyxes cut from agate-nodules are usually stained artificially, as explained under Agate.

The onyx is largely used for beads, brooches, pins, ring-stones and other small ornamental objects, while the larger pieces are occasionally wrought in the form of cups, bowls, vases, &c. Onyx is the favourite stone for cameo work, advantage being taken of the differently-coloured layers to produce a subject in relief on a background of another colour. For fine examples of ancient cameo-work in onyx and sardonyx see Gem.

It should be noted that the term onyx, or onychite, was formerly, and is still sometimes, applied to certain kinds of banded marble, like the “oriental alabaster” (see Alabaster). Such substances are quite distinct from the hard siliceous onyx, being much softer and less precious: they are, in fact, usually deposits of calcium carbonate like stalagmite and travertine. The ornamental stones known as Mexican onyx, or Tecali marble, and Algerian onyx are of this character; and in order to avoid any confusion with the true onyx it is well to distinguish all the calcareous “onyxes” as onyx-marble. The well-known “Gibraltar stone” is an onyx-marble, with brown bands, from caverns in the limestone of Gibraltar. The Tecali onyx, sometimes with delicate green shades, takes its name from the district of Tecali; one of its localities being La Pedrara, about 21 m. from the city of Puebla.

For onyx-marbles see Dr G. P. Merrill, Rep. U.S. Nat. Mus. for 1893 (1895), p. 539.  (F. W. R.*)