Usually (incorrectly) referred to simply as “pearl”, it is the nacreous material an oyster deposits on the inside of it’s shell, which can be cut and polished to a glass-like luster.  It often displays iridescent colors and fascinating patterns that have tremendous “eye appeal.”  Through the ages it has been the premier handle material on high quality knives.  It is brittle, easily damaged, and breathing the dust generated by cutting it causes lung disease.  As a result there have been many attempts to create synthetic substitutes for MOP.  Many of the knives advertised as having MOP handles are in fact handled with some form of plastic. Abalone and green sea snail are types of mother-of-pearl. See: Pearl