352 THE SEAS 



pearl essence usually mixed with celluloid. For the best 

 imitation pearls the pearl essence has to be chosen very 

 carefully because the effect produced depends on the size 

 of the guanin crystals. 



It is comparatively easy to distinguish an imitation from 

 a real pearl. The hollow glass imitation pearls can be 

 detected by the sharp reflection given by the glass surface 

 and by their lightness. The solid variety are not so easy 

 to detect, but the pearly coating can be cut off, is inflam- 

 mable, and can be dissolved with amyl acetate or acetone, 

 while the coating does not extend evenly up to the edge 

 of the hole through which the string passes. The genuine 

 pearl has a definite weight for its size and is somewhat 

 iridescent ; it gives no sharp reflection from its surface 

 and the string hole has clearly been drilled. Acetone and 

 amyl acetate have no effect upon its surface which is, 

 however, unlike those of the imitation varieties, attacked 

 by acid, since the pearl is made of calcareous matter. 



Precious Corals 



The beautiful red coral of commerce (Corallium nobile) is 

 quite unlike the " stony " corals which constitute the great 

 mass of coral reefs, being a " false " coral more nearly 

 allied to the Dead-men 's-fingers and sea fan so common in 

 our own waters. The red coral substance is not exposed 

 in life but forms the central supporting framework of the 

 colony, on the surface of which is a soft crust through which 

 ramify the canals which connect the flower-like white 

 feeding polyps with which the surface is dotted (Fig. 65). 



Red coral is found especially in the Mediterranean, off 

 the south of France and around the coasts of Corsica, 

 Sardinia, and Sicily and also along the north coast of Africa 

 from the Straits of Gibraltar to Tunis (Plate 122) . It spreads 

 into the Atlantic to some extent and is fished off the Cape 



