A pearl is a lustrous concretion produced by certain bivalve molluscs (muscles and oysters) and valued as a gem. Pearls consist almost entirely of nacre, which is the substance forming the inner layers of the mollusc shells. Nacre, known as mother-of-pearl, is composed primarily of aragonite crystals. The pearl is an abnormalgrowth resulting from the invasion of the body of the mollusc by a minute parasitic worm (it was previously believed to be invasion by a grain of sand or similar which caused the pearl to be produced), which cannot escape and irritates the mollusc which retaliates by coating the irritant with layer upon layer of nacreous material. Both marine and freshwater molluscs produce pearls, but the most valuable varieties originate in the pearl oyster of the Persian Gulf. The most highly prized pearls are spherical. When a pearl that has been cut from the shell presents a hemispherical surface, it is sometimes called a bouton pearl. If a solid pearl has an irregular shape, having grown over a rough object, it is known as a baroque pearl. In the jewellerytrade, pearls are commonly known as pear, bell, or drop, according to the shape. Pearl coloration varies widely, the most prized shades being white, black, rose, and cream. River pearls are produced by freshwater mussels in various parts of the world. China is the principal trader in river pearls. Natural, spherical pearls have been cultured successfully since 1920. In this process a mother-of-pearl bead, from three-quarters to nine-tenths of the diameter of the desired product, is introduced into the
pearl oyster. Over a period of years the oyster deposits layers of nacre around the bead. Cultured pearls are not easily distinguished from genuine pearls except by an expert. The technique of producing spherical cultured pearls was developed in Japan, and the culturing of pearls is a major Japanese industry - a small bead of mother of pearl being inserted into the oyster which grows the pearl around the bead. Artificial pearls, in contrast to cultured pearls, are entirely man-made, largely of glass. Research Pearl
Gordon Allport was an American psychologist. He was born in 1897 and died in 1967. He developed a theory of personality that emphasised individual uniqueness, and was the editor of the 'Journal of Abnormal and Social Psychology' from 1937 to 1949. He wrote 'Personality' in 1937 and 'The Nature of Prejudice' in 1954. Research Gordon Allport
Ankylosis is an abnormaladhesion or immobility of the bones in a joint, caused by example by a direct joining of the bones, a fibrous growth of tissues within the joint, or surgery. Research Ankylosis
Arrhythmia is a disturbance of the normal rhythm of the heart. There are various kinds of arrhythmia, some benign, some indicative of heart disease. In extreme cases, the heart may beat so fast as to be potentially lethal and surgery may be used to correct the condition. Extra beats between the normal ones are called extrasystoles; abnormal slowing is known as bradycardia and speeding up is known as tachycardia. Research Arrhythmia
In psychotherapy, catharsis is the process of relieving an abnormal excitement by re-establishing the association of the emotion with the memory or idea of the event that first caused it, and of eliminating it by complete expression. Research Catharsis
Abbreviations
Research Abnormal
