Pearl
PEARL (Heb. פְּנִינָה, Gr. μαργαρίτης, G3449) a calcareous concretion formed as an abnormal growth within the shell of some species of molluscs. The concretion is made generally of the mineral aragonite (calcium carbonate), or rarely of calcite (also calcium carbonate), together with the organic substance known as conchiolin. The microscopic crystals of aragonite are deposited on and around a tenuous network formed of the conchiolin. Genuine pearls result from the accidental entry of a grain of sand, or a parasite, into the pearl oyster which coats such a source of irritation with nacre (mother-of-pearl).
Pearl is the only gem made by a living process and the only one that comes from the sea. Pearls used to be obtained in considerable number from the Red Sea, but now first ranking of any oriental pearls for superior form (drop-like) and luster (iridescent) are those produced by Mohar, a variety of the Meleagrina vulgaris species of mollusc found in the Persian Gulf.
An unblemished pearl is one of the most ancient symbols of perfection and was among the most precious of gems (Matt 13:45, 46). This is prob. the reason why the word is used metaphorically for anything of great value, esp. wise sayings.
Bibliography
E. L. Young, “Pearl,” EBr 17 (1970), 505, 506.