PEARLS 587 



Mobius, the author of a valuable work entitled " The True Pearls," consider the formation 

 of pearls to be due most frequently to the irritation caused by the presence of a grain of 

 sand. Of fifty-nine pearls examined by Mobius himself, and obtained both from sea and 

 from fresh-water molluscs, a certain number were found to possess a nucleus consisting of a 

 crystalline, granular, calcareous substance, but that of the majority was brown in colour 

 and of organic origin, being possibly the remains of small intestinal worms, while in no 

 single case was the nucleus found to consist of a grain of sand. Not only is the formation 

 of pearls actually caused by the presence of parasites in the mollusc, but also by the attacks 

 of water-mites, small fishes, boring sponges, and worms, which penetrate the shell from 

 without, by the growth of algae, and even by the eggs of the mollusc itself. 



Pearls formed by difi"erent parts of the mantle often differ in shape and appearance. 

 For example, a pearl formed in the inside of the soft part of the mantle, perhaps in 

 response to the irritation caused by a parasite of some kind, will be more or less perfectly 

 spherical, and \\ill lie free in the mantle. If, however, the pearly matter is laid down 

 around the orifice made by some boring external parasite, the result will be a wai't-like 

 protuberance. In the former case is produced a pearl proper, which can be used as an 

 ornament without preliminary treatment ; in the latter, the so-called button or zcart-pearls, 

 which are very irregular in shape, but, notwithstanding this, are detached from the shell and 

 utilised in various ways as fantasy-pearls. In exceptional cases these button-pearls are 

 hollow and contain a beautiful spherical pearl, lying freely in the cavity. 



The immber of pearls found in a single mollusc varies according to circumstances. If 

 the mantle is in-itated at one spot only, then only one pearl is formed ; if at several spots, 

 then several pearls Mill be formed ; and in exceptional cases a single shell may contain a 

 large number. Among remarkable cases of this kind is that of a pearl-oyster from the 

 Indian Ocean, which contained eighty-seven pearls of good quality ; ^^hile in another from 

 Ceylon sixty-seven of various sizes were found. As a general rule, the gi'eater the number 

 of pearls found in a single shell the smaller they are. 



The first person to point out that the structure of pearls is identical with that of the 

 molluscan shell was the French naturalist Reaumur (1683-1757). This structure can be 

 best made out by examining under the microscope a thin section of a pearl taken right 

 through its centre. It will be seen to be built up of concentric coats like an onion. Each 

 of these concentric layers consists of overlapping laminae, exactly like those which build up 

 the mother-of-pearl layer in the shell of the mollusc. This concentric structure points to 

 the fact that the secretion of calcium carbonate was not continuous, but that there were 

 longer or shorter intervals of rest, coinciding, perhaps, with certain seasons of the year, 

 when growth also was arrested. Thus, each concentric layer corresponds to a period of 

 growth, and each interruption between neighbouring layers to an interval of rest. These 

 concentric coats are sometimes visible to the naked eye, but, as a rule, they are of microscopic 

 thickness. When a pearl is raised to a red-heat, the concentric coats peel off and the 

 laminae separate from each other. Much the same thing happens in the case of pearls 

 which have been worn for a long time strung together to form a necklace. At the spot 

 where each pearl is perforated, the concentric layers begin to fall off as scales, so that the 

 orifice through which the string passes becomes gradually wider and wider. The substance 

 of pearls has a hardness of between 3 and 4 on Mohs' scale, so that being comparatively 

 soft they cannot be expected to resist a large amount of hard wear. 



The majority of pearls, and certainly all the most beautiful, are formed in this way, 

 and with the exception of the nucleus consist wholly of layers of nacre. Not infrequently, 

 however, pearls are met with in which the nucleus is surrounded by a dark layer corresponding 



