mother-of-pearl (n.)

"nacreous inner layer of the shell of various bivalve mollusks," c. 1500, translating Medieval Latin mater perlarum, with the first element perhaps connected in popular imagination with obsolete mother (n.2) "dregs." Compare Italian madreperla, French mère-perle, Dutch parelmoer, German Perlmutter, Danish perlemor. It is the stuff of pearls but in a layer instead of a mass.