Onyx (ὄνυξ, a finger-nail). (1) A general name for the varieties of the agate which consist of alternate layers of white, brown, or black, greatly valued by the ancients for cameos. In the Christian symbolism the onyx typifies innocence and candour. (See Onicolo.) (2) The name has also been applied by the ancients to Oriental alabaster. (3) Onyx marble was a name given to Algerian marble from Oran, of which “pure white, brilliant red, golden yellow, and hues of green, with every variety of striation and flocculence, exist.” [See the Building News, xiv. 489.]
Opa, Opê, Gr. Arch. (ὀπή). A cavity in which a tie-beam (tignum) rests; whence the space included between two ὀπαὶ or tigna was called metopa or intertignum.
Opacity. Want of transparency.
Opaion, Gr. Arch. The panels on a ceiling formed by the intersection of its beams.
Opal. A semi-transparent stone, remarkable for the play of colours that it exhibits. Three varieties are, the oriental opal, called also the noble opal and the harlequin opal, remarkable for its flashes of brilliant colours having a triangular disposition. The affection that the ancients entertained for this beautiful gem was unbounded. The Roman senator Nonnius preferred exile to parting with a brilliant opal the size of a filbert which Marc Antony coveted. The fire opal is furnished principally by Mexico. Its colour, more pronounced than that of the oriental opal, and the carmine or vinous red tint of its fires, permit it to be easily recognized. The common opal displays very little fire; its colour is milk-white, which, joined to a texture extremely homogeneous, renders it semi-transparent. [L. Dieulafait.]
Opal Glass, called also Milk-white Glass; prepared for globes to lamps, &c.
Opales, Opalia, R. Festivals of Ops, the wife of Saturn, which were held every year on the fourteenth of the calends of January (19th of December).
Opalescent. Having a play of colours like the opal.
Open-tide, O. E. The season between Epiphany and Ash-Wednesday, when marriages were publicly solemnized.
Opera. A lyrical drama set to music; originated at Florence in the 16th century. [Consult Doni (passim), Arteaga Manfredini, Signorelli, &c.; also Dr. Burney’s Tours and Correspondence, and Grimm’s Correspondence.]