Opal
a precious stone of various colors, which comes under the class of pellucid gems. It consists of silica with about 10 per cent. of water, and is very brittle. It is characterized by its irides- OPERA cent reflection of light. It is found in many parts of Europe, especially in Hungary, in the East Indies, etc. The substance in which it is generally found is a ferruginous sandstone. There are many varieties or species, the chief of which are (a) precious or noble opal, which exhibits brilliant and changeable reflections of green, blue, yellow, and red; (b) fire opal, which simply affords a red reflection; (c) common opal, whose colors are white, green, yellow, and red, but without the play of colors; (d) semiopal, the varieties of which are more opaque than common opal; (e) hydrophane, which assumes a transparency only when thrown into water; (f) hyalite, which occurs in small globular and botryoidal forms, with a vitreous luster; (g) menilite, which occurs in irregular or reniform masses, and is opaque or slightly translucent.