A shelly concretion, usually rounded, and having a brilliant
   luster, with varying tints, found in the mantle, or between the mantle
   and shell, of certain bivalve mollusks, especially in the pearl oysters
   and river mussels, and sometimes in certain univalves. It is usually
   due to a secretion of shelly substance around some irritating foreign
   particle. Its substance is the same as nacre, or mother-of-pearl.
   Pearls which are round, or nearly round, and of fine luster, are highly
   esteemed as jewels, and compare in value with the precious stones.