66 GENERAL CHARACTERS OF PRECIOUS STONES 



since it feels so much warmer to the touch. Another substance having a feeble conductivity 

 for heat is jet, a variety of coal, which is frequently made use of for mourning ornaments. 

 An opaque, black glass is often used for the same purpose, but a single touch of the finger 

 tips is all that is needed to enable an expert to distinguish between the two. 



A device for distinguishing a genuine from an imitation stone, depending upon the 

 power of conducting heat, is to breathe upon the stone. The moisture of the breath will 

 condense upon the genuine stone with more difficulty than upon glass, and when condensed 

 will disappear again much more rapidly, since the precious stone is both more rapidly 

 warmed and more rapidly cooled than is glass. 



For the purpose of distinguishing rough stones, their fusibility before the blowpipe 

 may sometimes be made use of. All glass imitations are easily fusible before the blowpipe, 

 while few of the minerals used as precious stones can be so fused. Red garnet is one such 

 mineral, and can be easily distinguished from other red stones which are infusible before the 

 blowpipe, such as ruby and spinel. The application of this test is naturally limited to rough 

 stones, splinters of which can usually be detached for examination. 



2. Electrical Chakactees. 



Many precious stones when exposed to certain external influences acquire a greater or less 

 charge of electricity. They differ from each other in the length of time this charge can be 

 retained, some retaining it for a considerable time, others for a less time, perhaps only a few 

 minutes. 



The French abbe, Haiiy, the founder of modern scientific mineralogy, attempted to 

 make extensive use of these characters as a means of identifying stones and distinguishing 

 them one from another. In his book, published in 1817, Traite des caracteres physiqiies 

 des pierres precieiises, he devoted seventy- two out of a total of two hundred and fifty-three 

 pages to the consideration of electrical characters, while the optical characters are dismissed 

 in thirty-two pages. A comparison with the number of pages devoted to the treatment of 

 these two branches in the present volume, shows how much more important to-day is the 

 consideration of the optical characters of minerals. 



The examination of the electrical, as of the optical, characters of a stone, has the 

 advantage that no injury to the stone results therefrom. The observation of electrical 

 characters, however, requires a certain amount of skill and practice ; for the detection of 

 the very small electrical charges acquired by most precious stones is difficult ; and, further, 

 these observations must be conducted in a perfectly dry atmosphere, a condition not 

 always easy to obtain. Any charge located on the surface of a stone is rapidly lost in 

 a damp atmosphere, and a stone which retains its charge in dry surroundings will rapidly 

 lose it in the presence of moisture. The length of time a stone retains its charge, 

 a test to which Hatty attached great importance, depends largely therefore upon 

 external conditions. 



At the time Haiiy was engaged on his researches the methods of electrical investiga- 

 tion were, at least for his purposes, fairly well developed, while methods for the optical 

 investigation of minerals had received little or no attention. Observers had indeed noticed 

 that some minerals were singly refracting and others doubly refracting, but there was no 

 polariscope to give precision to their observations and the phenomenon of dichroism had yet 

 to be discovered. We can thus readily understand why Hatty attached so much more 

 importance to the electrical than to the optical characters of minerals and precious stones. 

 With the discovery of the dichroscope and a convenient polariscope the optical characters 



