12 GENERAL CHARACTERS OF PRECIOUS STONES 



exception, perhaps, of the optical characters, no other feature is as important as a means 

 of determination. 



Every dealer in jewels should therefore be able to make a rapid determination of the 

 specific gravity of any given gem with sufficient accuracy for practical purposes. The time 

 expended in acquiring the necessary manipulation and the cost oi the apparatus will be amply 

 rewarded. It is proposed to give here a detailed account of the methods in use for the 

 determination of specific gravity, not only for those conspicuous for the accuracy of the 

 results they furnish, but also such as would be chosen when rapidity andeaseof manipulation 

 rather than extreme accuracy are needed. 



It will be noticed that the value of the specific gravity of any given mineral, as quoted 

 in text-books or works of reference, varies between certain narrow limits. This is due, 

 firstly, to small errors in determination ; and, secondly, to the impossibility of obtaining two 

 absolutely identical specimens of the same mineral, since there may be, in certain cases, small 

 or even considerable variations in chemical composition (isomorphous replacement) ; and, 

 again, crystals frequently contain as enclosures various amounts of difl^erent impurities. 



Methods for the Determination of Specific Gravity. 



1. Method with the Pycnometer. — This is perhaps the most accurate of the several 

 methods available for the determination of specific gravity. The pycnometer or specific 

 gravity bottle (Fig. 1) is a small vessel of thin glass with a wide 

 neck, into which fits a ground-glass stopper. The stopper is per- 

 forated longitudinally by a very fine canal. 



To find the specific gravity of a stone its weight g is first 

 observed. Next the weight p of the specific gravity bottle, filled with 

 distilled water, is found ; this observation can be made once for all 

 (for the same flask), and need not be repeated at each new determi- 

 nation. Care must be taken that the water rises to the top of the 

 perforation in the stopper; this is usually effected by filling the 

 flask quite full and then inserting the stopper. Before weighing, 

 the flask must be well dried on the outside. The stone is now placed 

 Vactua/sizer^ ^^ ™ ^^ flask. It displaces an amount of water equal in volume to its 

 own bulk. The stopper is replaced so that water rises to the top of 

 the perforation. The flask must be again dried and weighed. Let this weight be denoted 

 by q. The weight of the flask, full of water, together with the weight of the stone is 

 g + p- The weight of water displaced by the stone in the second part of the operation is 

 consequently g + p — q, and this represents an amount of water equal in volume to the 

 volume of the stone. Since then the weight of the stone is g, and the weight of an 

 equal volume of water ^^ g + p — q, the specific gravity, d, is therefore given by 



d= §- . 



g + p - q 



To take a numerical example; let the weight of a stone be ^=4!'382 in grams or 

 whatever unit is used ; the weight of the bottle filled with water be jt) = 15-543 ; the weight 

 of the bottle containing both the stone and the water g' = 18"680. Then the weight of the 

 displaced water \s g + p - q = 4-382 + 15-543 - 18-680 = 1-245 and 



_ g 4-382 _ „ _, 



g + p - q~ 1-245 - ^■^^■ 



This is the specific gravity of the stone, in this case a topaz. 



