^4 GENERAL CHARACTERS OF PRECIOUS STONES 



simply a means to an end, namely, the discrimination of stones similar in appearance and the 

 identification of others. Hence it is often only necessary to ascertain whether the specific 

 gravity of any particular stone exceeds a certain value. As an example, let us suppose 

 that a doubt exists as to whether a colourless stone is rock-crystal (quartz, sp. gr. = 2-65) or 

 topaz (sp. gr. = 3-5) ; by simply placing it in methylene iodide (sp. gr. = SS) the doubt is 

 settled at once, for if rock-crystal it will float, but if topaz it will sink. 



Methods such as this can be very advantageously used for the rapid discrimination of 

 precious stones. It is necessary, however, to be provided with a series of liquids of various 

 known densities in which the stone under examination may be dropped. The approximate 

 value of its specific gravity can be learnt by simply observing its movement upwards or 

 downwards in these liquids. As has been pointed out before, a slow movement up or down 

 indicates that the difference in specific gravity between the stone and the liquid is small ; a 

 quick movement indicates a greater difference ; while an absence of movement shows that the 

 specific gravity of the stone is identical with that of the liquid. 



In practice a good series is furnished by the following four liquids : 



No. 1. Methylene iodide saturated with iodine and iodoform. Specific gravity = 3-6. 



No. 2. Pure methylene iodide. Specific gravity = 3-3. 



No. 3. Methylene iodide diluted with benzene. Specific gravity = 3-0. 



No. 4. Methylene iodide further diluted with benzene. Specific gravity = 2-65. 



The liquids of this series may be numbered consecutively from the heaviest to the 

 lightest, and will be frequently referred to below as liquids No. 1, No. 2, No. H, and No. 4-. 



With such a series of liquids it is possible to make an approximate determination of 

 the specific gravity of almost any precious stone with great ease and rapidity, and this will 

 be of considerable aid in its recognition. The stone under observation should be first 

 placed in liquid No. 1 ; should it sink, which can be easily seen in spite of the deep colour 

 of the liquid, its specific gravity is greater than 3-6. If, on the other hand, it rises to the 

 surface, it must be removed with a pair of forceps, wiped with a cloth moistened with 

 benzene, and placed in liquid No. 2. Should it sink in this liquid, we then know that its 

 specific gravity lies between 36 and 3*3 ; if it remains suspended then its specific gravity is 

 exactly 3*3. If, on the contrary, it floats to the surface, it must be taken out and placed 

 successively in liquids No. 3 and No. 4, and similar observations made. When a stone 

 slowly sinks or slowly rises in either of the liquids we know that its specific gravity is slightly 

 .greater or slightly less than that of the liquid. A stone of specific gravity 3'02, for example, 

 would slowly sink in liquid No. 3. 



The series of liquids for such determinations should be kept ready to hand in wide- 

 mouthed glass bottles properly labelled and closed with ground-glass stoppers. The latter 

 precaution is to avoid evaporation of the methylene iodide, which is somewhat expensive 

 (four shillings per ounce), and of the very volatile benzene, and thus prevent alterations in 

 the specific gravity of the liquids. When not in use the bottles should be kept in a dark 

 place, since exposure to light causes methylene iodide to slowly decompose with separation 

 of iodine which darkens the liquid. When after long use, methylene iodide has become dark 

 in colour it may be decolourised by shaking it up with a dilute solution of caustic potash, 

 which must afterwards be poured off' or removed by means of a separating funnel. 



The specific gravity of the four standard liquids is very liable to change, partly on 

 account of evaporation and partly on account of the small quantities of liquid introduced 

 with the stones if they are not well dried after immersion, Hence the specific gravity of 

 each of the four liquids must be frequently checked by means of WestphaPs balance or, 



