in natural history, a species of the chroa-places genus of gems.

The opal is a gem of a very peculiar kind, and has been esteemed by many in all ages of very great value; though at present it is of less value, in proportion to its size, than any of the finer gems. It is softer than any other of the fine gems, and is difficult to polish to any degree of nicety. It is found of various shapes and sizes; its most frequent bigness is between that of a pea and a horse-bean; but it is found as small as the head of a large pin, and has been seen of the size of a large walnut. Its figure is very various and uncertain, but it is never found in a crystalliform or columnar state; its most usual shape is an irregular oblong one, convex above, flatted at bottom, and dented with various finuositites at its sides. It is often found among the loose earth of mountains, sometimes on the shores of rivers, and not unfrequently bedded in the coarser kinds of jasper. It is found in Egypt, Arabia, some parts of the East Indies, and in many parts of Europe: those of Europe are principally from Bohemia, and are of a greenish or greyish colour; the colour of the other opals much resembles the finest mother of pearl, its basis seeming a bluish or greyish white, but with a property of reflecting all the colours of the rainbow, as turned differently to the light.