564 DETERMINATION OF PRECIOUS STONES 



dichroism has the advantage of being applicable to stones which are mounted d,jour as well 

 as to cut and rnicut unmounted stones. 



In the possession of every person having to do with precious stones there should be a 

 dichroscope, a polariscope of the kind described and figured above, and a series of heavy 

 liquids with indicators, or, in place of these, a Westphal's balance. No special skill or 

 theoretical knowledge is required in the use of these instniments, and their judicious employ- 

 ment constitutes a safeguard against the many cases of loss which arise through errors of 

 determination. 



It is desirable or necessary at times to take into consideration other characters of the 

 stone, the determination of which may possibly result in a slight injury. Thus it is 

 permissible to determine the hardness of a single cut stone, but tests for the fusibility or the 

 behaviour towards acids can only be applied when a parcel of rough stones is at the disposal 

 of the experimenter. In buying a parcel of gems it often pays the merchant to sacrifice one 

 .stone in order that an exhaustive determination, including perhaps a chemical analysis, may 

 be made of it. In the case of rough stones it is often possible to detach a small splinter for 

 detailed examination, and a slight scratch, or spot marked by acids, is of no consequence, 

 since it will be removed when the stone is cut. 



Minute differences in hardness cannot be relied upon for the purpose of discriminating 

 stones, since differences of the same degree often exist between different faces of a crystal, or 

 in different directions on the same face. The determination of the hardness of a stone is 

 best performed by drawing a projecting edge or corner across the smooth surfaces of the 

 different minerals which constitute the scale of hardness. We have already seen, in the 

 earlier part of this work, that for the determination of the hardness of all ordinary precioug 

 stones pieces of only three minerals are necessary, namely, felspar (H = 6), quartz (H = 7), 

 and topaz (H =8 ), the pieces of which should have smooth faces and preferably should be 

 artificially polished. No. 5 of the scale may be replaced by a piece of glass, lower members, 

 as also Nos. 9 and 10, are not often needed. In the case of cut stones the edge with which 

 the scratch is made should be chosen at the girdle, which will be enclosed and hidden within 

 the setting. But even with this precaution the operation must be performed with great 

 care, for the pressure may cause the stone to splinter at the edge, especially when it possesses 

 a perfect cleavage. 



It is necessary sometimes to reverse the operation and to attempt to scratch the stone 

 itself. The instrument employed by preference in such cases is a pencil of specially 

 hardened steel, provided with a sharp point, which will scratch quartz slightly and glass 

 easily. It is easy in this way to distinguish glass imitations from genuine precious stones, 

 such as the diamond, ruby, sapphire, &c., which are not scratched by the steel point, and the 

 damage caused to the worthless imitation is not of very great consequence. Exti-a care is 

 only necessary when dealing with the softer precious stones, such as chrysolite, &c. ; but it 

 is advisable in all cases to choose a spot on which to apply the test, which will be hidden 

 when the stone is mounted. A scratch on almost any part of a transparent stone detracts 

 considerably from its beauty, but a scratch on the back of an opaque stone is of little conse- 

 quence. A rough stone may be scratched anywhere without fear, and the hardness of 

 different faces and in different directions of the same face determined. 



Instead of the steel pencil a file of hard steel is frequently employed ; this readily marks 

 the softer stones, and emits at the same time a lower note than with harder stones. Cut 

 stones should be tested with the file only on the girdle, and even then with the greatest 

 care. Owing to all these difficulties only a limited use can be made of the character of 

 hardness. Recently, however, a very good method, depending upon hardness, has been 



