Opal

(o'pul.) [L. opal us.] (Min.) A precious stone of various colors, which comes under the head of pellucid gems. It consists of silex, with abt. ten per cent. of water. It is found in many parts of Europe, especially in Hungary. It is brittle, and when first dug out of the earth is soft; but it hardens and diminishes in bulk by exposure to the air. The substance in which it is most commonly found is a ferruginous sandstone. It is generally dull, owing to foreign admixture; but in some specimens a lively play of light is observable, while others show different colors by reflected and transmitted light. [s. 664]