562 DETERMINATION OF PRECIOUS STONES 



The specific gravity is best and most conveniently determined by the aid of certain 

 heavy liquids, especially methylene iodide, which can be diluted to any extent with 

 benzene until the stone under observation remains suspended in the liquid. The specific 

 gravity of the liquid, and therefore of the stone, can then be determined by Westphal's 

 balance (Fig. 7), or more conveniently by means of indicators, such as fragments of various 

 minerals, of known specific gravity. In the case of stones which sink in pure methylene 

 iodide, the specific gravity of which at ordinary temperature is 3'3, a heavier liquid, 

 obtained by saturating methylene iodide with iodine and iodoform, may be used ; or their 

 specific gravity may be determined by the use of the pycnometer, the hydrostatic balance, 

 or Westphal's balance (Fig. 5). Stones with a specific gravity exceeding S6 will sink even 

 in this heavier liquid, and for them a still heavier liquid, molten silver-thallium nitrate, 

 must be used. For ordinary purposes an exact determination of the specific gravity is 

 unnecessary, and the four standard liquids recommended above will be found quite sufficient. 

 These being so frequently referred to may be repeated here : 



No. 1. Methylene iodide saturated with iodine and iodoform. (Sp. gr. = 3'6.) 

 „ 2. Pure methylene iodide. (Sp. gr. = 3"3.) 

 „ 3: Methylene iodide dUuted with benzene. (Sp. gr. = 3'0.) 

 „ -4. Methylene iodide further diluted with benzene. (Sp. gr. = 2"65.) 



The data furnished by the use of these liquids enable us to classify precious stones into 

 five groups, a classification which, taken in conjunction with the other characters of a stone, 

 is a valuable aid in determining its identity. 



The specific gravity method of determination is just as applicable to rough as to cut 

 stones ; the former when under determination should be free from all adhering foreign 

 matter, and the latter should of course be unmounted. The merest fragment of a stone, 

 with a density less than SS or SG, will float in one or other of the heavy liquids, and thus 

 indicate approximately what its specific gravity is; but for the exact determination of 

 heavier stones, unless silver-thallium nitrate (sp. gr. = 4-8) is used, a rather larger fragment 

 will be needed. 



In establishing the identity of a stone next in importance to the specific gravity come 

 the optical characters. The chief of these for our present purpose is the character of 

 the refraction, whether double or single. This may be determined sometimes by direct 

 observation, for, with cut stones, each facet forms with the one next the eye a prism, and 

 the image of a small flame given by this prism will be single in the case of singly refracting 

 stones (Fig. 266) and double with doubly refracting stones (Fig. 26m). In the case of 

 feebly doubly refracting stones the two images overlap so much that they appear to be one, 

 so that it is not possible by these means to distinguish such stones from stones which are 

 singly refracting. Moreover, observations of this nature can only be made on stones which 

 are perfectly transparent and bounded by smooth crystal-faces or plane polished facets; 

 irregular broken fragments, or stones cut en cabochon, will not give sharp imao'es of the 

 flame. 



For these reasons, therefore, it is necessary in some cases to resort to the polariscope 

 (Fig. 27) for aid in the determination of the nature of the refraction of a stone. The stone 

 is placed on the object- carrier of the instrument with the planes of polarisation crossed, 

 that is to say, in the dark field. If during the course of a complete revolution of 360° the 

 stone changes from light to dark four times it is indisputably doubly refracting. If, on 

 the other hand, the stone remains during a complete rotation of the carrier uniformly dark, 

 like the rest of the field of view, it may be singly refracting. This, however, is not sufficient 



