Do you know ghouls from goblins and ghosts?
c.1500, translating Medieval Latin mater perlarum, with the first element perhaps connected in popular imagination with obsolete mother (n.2) "dregs." Cf. Italian madreperla, French mère-perle, Dutch parelmoer, German Perlmutter, Danish perlemor.
The hard, smooth, pearly layer on the inside of certain seashells, such as abalones and certain oysters. It is used to make buttons and jewelry. Also called nacre.