British Dictionary definitions for mother of pearl
a hard iridescent substance, mostly calcium carbonate, that forms the inner layer of the shells of certain molluscs, such as the oyster. It is used to make buttons, inlay furniture, etc Also called nacre, related adjective nacreous
1510, translating M.L. mater perlarum, with the first element perhaps connected in popular imagination with obsolete mother "a thick substance concreting in liquors; the lees or scum concreted" [Johnson], which is from the root of mud.