98 THE WEALTH OF THE SEA 



with the nacreous portion of the pearl shell of the 

 oysters in which they are found. Both contain about 

 ninety-two per cent of calcium carbonate, six per 

 cent of organic matter, and two per cent of water. 

 Their density is about 2.^5 times that of water, 

 and their hardness is comparable to that of fluorite. 

 Since pearls consist largely of calcium carbonate, 

 they are easily attacked and destroyed by all the 

 strong acids. Even some relatively weak acids have 

 a slight action. For instance, there is sufficient acid 

 in the perspiration of certain individuals to affect 

 their luster. Because of the relative softness of pearls, 

 they must be kept from contact with precious stones. 

 A pearl ring should never be worn on the same finger 

 with a ring set with a diamond, ruby, or other stone 

 setting. No abrasive should ever be used in cleaning 

 or polishing pearls. Likewise, since chemicals may 

 stain or dull them, no cleansing agents of unknown 

 composition should ever be used on them. Ordinarily 

 soap and water or dilute alcohol should be used in 

 cleansing pearl necklaces or other pieces of jewelry 

 containing pearls. Pearls should never be left wet, 

 nor should they be stored in an absolutely dry 

 atmosphere. When pearls are stored in a safe-deposit 

 vault, moist blotting-paper is sometimes placed in 

 the box so as to prevent them from becoming thor- 

 oughly dried out. 



The Fisheries 



Since the only shells which yield valuable pearls 

 are those containing mother-of-pearl, and but a small 

 proportion of these contain pearls, pearl fisheries are 



