GEMS 629 



only scientific resources that remain are the trial by electricity, which is often inconclu- 

 sive ; the degree of hardness, a criterion requiring great experience in the person who 

 employs it ; and lastly, the proof of specific gravity, unquestionably one of tho surest 

 means of distinguishing the really fine gems from ornamental stones of similar colour. 

 This proof can be applied only to a stone that is not set. 



The specific gravity may bo determined by any person of common intelligence, 

 with the aid of a -small hydrostatic balance. If, for . example, a stone of a fine 

 crimson-red colour be offered for sale as an oriental ruby, the purchaser must ascertain 

 if it be not a Siberian tourmaline, or a spinel ruby. Supposing its weight in air to be 100 

 grains, if he finds it reduced to 69 grains when weighed in water, he concludes that its 

 bulk is equal to that of 31 grains of water, which is its loss of weight. Now, a real 

 sapphire which weighs 100 grains in air, would have weighed 76'6 in water ; a spinel 

 ruby of 100 grains would have weighed 72'2 in water, and a Siberian tourmaline of 

 100 grains would have weighed only 69 grains in water. The quality of the stone in 

 question is, therefore, determined beyond all dispute, and the purchaser may be thus 

 protected from fraud. See DIAMOND, ETJBY, &c. &c. 



As the discrimination of one species of gem from another is often a matter of great 

 practical importance, and sometimes of much delicacy, it may be useful to group to- 

 gether such easily-applied tests as will enable even an inexperienced observer to 

 determine the true character of any given stone. In the following comparative tables 

 the specific gravity (S.G-.) is placed in the first column, as being in many cases the most 

 valuable characteristic. This may be determined by means either of the hydrostatic 

 balance or of Nicholson's hydrometer. For determining the loss of weight in water, 

 a delicate chemical balance is generally needed ; a common pair of grain scales is 

 certainly of no use when operating on specimens weighing less than 10 grains. 

 Distilled water should be employed; but, in its absence, rain-water may be used. 

 With great experience, a rough notion of the specific gravity of a stone may be obtained 

 by poising it in the palm of the hand. In the second column of the table the degree of 

 hardness (H.) is specified. These relative degrees of hardness are expressed on Mohs's 

 mineralogical scale. If the specimen under test be too small to be conveniently held 

 in the fingers, it may be partially embedded in a stick of sealing-wax, and examined in 

 this position. A glazier's diamond is sometimes useful, as it scratches every gem-stone, 

 excepting tho diamond ; whilst a good eteel file is serviceable in distinguishing true 

 stones from pastes. The third column of the tables gives the character of the refrac- 

 tion, whether single (S.E.) or double (D.E.), which each gem exhibits. This character 

 may be ascertained by placing the stone between two crossed plates of tourmaline ; if the 

 dark field becomes more or less illuminated, the mineral is doubly-refracting, whilst if 

 it remains dark, it is probably singly -refracting. Other optical characters of cut stones 

 may be examined by 'the dichroiscope. This instrument is of no service in dealing with 

 singly-refracting minerals, but with doubly-refracting species it gives two images, which 

 often differ more or less decidedly in tint. The crystalline form and chemical composi- 

 tion are not noted in the following tables, because they are not available characters in 

 examining cut gem-stones, but they are fully given in the articles which treat specially 

 of each gem. As colour is the most obvious characteristic of a stone, it is used as the 

 basis of arrangement in the following scheme. The colour of a cut stone is seen to 

 greatest advantage when the specimen is held under water, at a depth of about half an 

 inch from the surface. 



In order to determine the true colour of a mounted stone, it should be removed from 

 its setting ; for the natural colour is often modified either by placing a foil behind the 

 gem, or, occasionally, by painting the back of the stone itself. 



Colourless Gems. 



The colourless stones most likely to occur in commerce are the diamond, the topaz, 

 and rock-crystal. These may be distinguished by the following characters : 



Diamond. . . . . . S.G-. 3-5 H. 10 S.E. 



Topaz . ... . . . 8-5 ,,8 D.E. 



Kock-crystal . . . . . 2'5 ,,7 D.E. 



The diamond is distinguished from all other stones by its excessive hardness; 

 a glazier's diamond easily scratches topaz and rock-crystal, but does not touch, or scarcely 

 touches, another diamond. The topaz is distinguished, in addition to the characters 

 given above, by its perfect cleavage in one direction, so that a joint or flaw often runs 

 across the stone ; it also becomes electric when heated, or when a smooth face is ex- 

 cited by friction. r , 



In addition to these gems, the following colourless stones, though comparatively 

 rare, may sometimes, occur in trade : - 



