A potassium feldspar that occurs in schists (notably in the Swiss Alps), often with a distinctive opalescence, and is now regarded as a variety of orthoclase.
((n.) A transparent or translucent variety of common feldspar,
or orthoclase, which often shows pearly opalescent reflections; --
called by lapidaries moonstone.)
Origin:
Late 18th century; earliest use found in The Critical Review. From Italian adularia from Adula, the name of an area of the Alps east of the St Gotthard Pass + -ia, with insertion of -r- apparently for euphony.