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).