Onyx. A species of agate, is a precious stone, of a dark horny colour, resembling, as its name imports, the nail of a man's finger, in which is a plate of a bluish white, and sometimes of red; the several colours appearing as distinct as if laid on by art. There are some brought from Arabia, mixed with a browish hue ; which after taking off one layer of zone, shew another underneath of a different colour. White zones or girdles are essential to an onyx.