[Fr., from L, unio.] A genus of plants, which includes also the garlic, leek, and shallot. The onion is very extensively cultivated, and grows best in a rich and rather moist soil. Its root bears a round or oblong bulb, widely in use as a pot-herb and as a table food. It is very nutritious, and easily digested.

0'nyx. A variety of chalcedony with some resemblance to agate. It is made up of alternating parallel bands of different colors, and was used by the ancients in making cameos, the figures being cut in the white layers, while the darker layers formed the background.

0'pal. [L, opalus.] A mineral consisting of silica with a small admixture of alumina. Precious opal presents a fine play of colors, due to its great number of minute fissures, and is highly esteemed as a gem for setting in rings, brooches, and other ornaments. The finest opal comes from Hungary and Mexico. Common opal is semi-transparent, white, yellow, green, red, or brown, and has no play of colors.