Show formation of pearls pearl A pearl Is formed by a 3 secretion that becomes pearly nacre the identical substance forming the inner lining of the shell around some foreign substance which has found its way into the shell but has not been aborted the intruder be it a grain of sand or a piece of shell causes irritation and d as a means of self protection rt a film filin of I 1 secretion Is laid upon lt it in it the e caie cas e I 1 of round pearls a pearl sac usually grows around the intruder and the inner surface of the sac secretes the fluid which gradually hardens and becomes a layer of pearl As succeeding layers form the pearl increases in size and value |