Something margaritaceous has an iridescent luster resembling that of a pearl or mother-of-pearl. A conch shell has a margaritaceous interior lining, as do oyster and abalone shells.
Margarita is the Latin word for "pearl," and the suffix -aceous means "having the nature of" or "resembling." So something margaritaceous resembles a pearl. The word is most often used to describe things in nature. An entomologist might use it to describe the shimmering wings of a dragonfly or the iridescent exoskeleton of a beetle. A botanist might use it to describe the satiny, silvery-white underside of certain petals or leaves. A more commonly used synonym for margaritaceous is nacreous.