Mother-of-pearl. Certain shellfish, such as pearl oysters, abalones, and freshwater mussels produce shells lined with a lustrous, rainbow-colored material. This material is called mother-of-pearl, or nacre. It varies in color from pale grayish-blue and pink to purple and green. Shells containing mother-of-pearl are found off the coasts of tropical countries, particularly around the South Sea Islands, the Philippine Islands, Australia, Panama, and Lower California.