PEARLS. 



THE NATURE AND FORMATION OP PEARLS.— We now come to the consideration 

 of pearls, those objects which, on account of their beauty and costliness, may well rank 

 next to the most splendid jewels. They are products of the life and activity of certain 

 molluscs, the insignificant-looking inhabitants of warm seas and of the rivers and streams of 

 many temperate regions; they thus differ in an important way from f)recious stones, but like 

 the latter have been used from time immemorial for every kind of decorative purpose. In 

 form they are spherical, ovoid, or pear-shaped,- or sometimes quite irregular; they are never' 

 transparent, but at most only tsanskicent ; they vary considerably in size, and, as a rule, are 

 colourless. Their beauty depends largely upon the peculiar lustre of their surface. Though 

 to appreciate this beauty it is by no means necessary to be familiar with the minute 

 structure and mode of origin of pearls, yet a knowledge of these is an aid to the 

 comprehension of their characteristic features. Before beginning the consideration of these 

 features, therefore, we will describe the mode of origin of the pearl. 



A substance which stands in the closest relationship to pearls, and which, moreover, 

 has derived from them its name mother-of-pearl, is also much used in decorative work. It 

 ha^ a lustre exactly similar to that of pearls, and frequently exhibits a more or less marked 

 and beautiful play of colours. It forms the inner coating of the shells of many bivalve 

 molluscs, and a mass of similar nature occurs also in the shell of certain univalve molluscs. 

 As the formation of pearls by univalve molluscs is a rare phenomenon, and without 

 significance from the point of view of pearl-fishers, we will confine our attention to the 

 pearl-forming bivalves. 



The thin outermost layer, the periostracum or so-called " epidermis," of the shell of 

 any bivalve mollusc, consists of a horny material known as conchiolin. Beneath this comes 

 the shell proper, consisting of two layers which differ in structure. The outer or prismatic 

 layer is formed of minute prisms of calcium carbonate separated by thin layers of conchiolin ; 

 the inner forms the internal part of the shell, and is built up of alternate layers of calcium 

 carbonate and conchiolin arranged parallel to the surface. The laminated internal layer is 

 what is known as nacre or mother-of-pearl, and varies in thickness in different molluscs. 

 The laminae of which it is built up consist of that variety of calcium carbonate which is 

 known to mineralogists as aragonite. The calcareous prisms, which form the middle layer 

 of the shell, consist, on the other hand, of calcite, another modification of calcium carbonate 

 which is softer and lighter than aragonite. 



The laminae, of which the pearly or nacreous layer of the shell is built up, are small 

 compared with the size of the shell, and overlap one another something like the tiles on the 

 roof of a house. This finely laminated structure is the cause of the peculiar lustre which 



