Mother of pearl (MOP)is the iridescent coating on the inside of mollusk shells. It is so named because when an irritant gets inside a shell, the shell protects itself by coating the irritant with the same material (nacre) of its lining that creates pearls. Therefore, this substance is the creator, or mother, of pearls! The finest "mother-of-pearl" is found in shells from Australia and Sri Lanka. Colored pearls and mother-of-pearl beads are dyed.