Opal, a variety of quartz, of which the finest kind, precious opal, is translucent, with blue or yellow tint, and when polished with a convex surface shows an admirable play of colours; it is found chiefly at Cerwenitza, Austria.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Oosterzee, Jan Jakob van * Open Secret, The