THERMAL CHARACTERS 65 



Finally, the phenomena of fluorescence and phosphorescence must be briefly described, 

 though they are of little importance in the case of minerals used as precious stones. 



Fluorescence is shown to a marked degree by fluor-spar from the lead mines of 

 Cumberland ; the phenomenon, indeed, takes its namelroni thismmeral, which is known to 

 mineralogists as fluor or fluorite. A fluorescent substance appears of one colour in transmitted 

 light and of quite another in reflected light ; thus fluor-spar from Cumberland is green in 

 transmitted light and purple in reflected light. This mineral is, however, very little used as 

 a precious stone ; one which is used more frequently for this purpose is amber, and specimens 

 from certain localities, namely from Sicily and Burma, show a remarkable fluorescence. 

 Amber from these localities varies in colour from yellow to brown in transmitted light, and 

 from green to blue in reflected light. The rounded polished surface of such specimens shows 

 a peculiar sheen, which, according to present tastes, diminishes the beauty of the stone, and 

 consequently its value. 



Substances in which the phenomenon of phosphorescence is seen emit, when 

 submitted to certain external influences, a soft, white or coloured light which is often 

 only distinctly visible in a dark room. In some cases the emission of light persists for some 

 time, while in others it lasts for a much shorter time, perhaps for only a few moments. 

 Phosphorescence is exhibited by several precious stones, and may therefore aid in their 

 recognition or discrimination. Two pieces of rock-crystal (quartz) phosphoresce when rubbed 

 one against another ; diamond shows a marked phosphorescence when rubbed on cloth ; even 

 when lightly rubbed on a coat-sleeve it will be seen in the dark to phosphoresce brilliantly. 

 Many diamonds also phosphoresce after being exposed to the direct rays of the sun ; they 

 store up the sunlight, as it were, in order to give it out again when placed in darkness. 

 Lapis-lazuli from Chili phosphoresces when warmed to a temperature considerably less than 

 that of red-heat ; white topaz, some diamonds, and other minerals behave in the same way. 

 In many minerals the phosphorescence induced by warming lasts only for a short time, but 

 may be produced again and again on reheating. 



E. THERMAL, ELECTRICAL, AND MAGNETIC CHARACTERS. 



There is nothing of special importance in the behaviour of precious stones when exposed 

 to the influence of heat, or in their electrical and magnetic characters. 



1. Thermal CnARAcrEEs. 



Different minerals differ very considerably in their conductivity for heat, and this 

 character may serve in some cases to distinguish minerals similar in appearance from each 

 other. The majority of precious stones are good conductors of heat, and on this account 

 they are cold to the touch, since the heat of the hand is quickly conducted away. Glass 

 is a somewhat poorer conductor and hence a glass imitation is not so cold to the touch 

 as a genuine stone, since the warmth of the hand is not so quickly conducted away. The 

 difference in the power of conducting heat of genuine stones and their imitations may thus, 

 under certain conditions, afford a means of distinguishing between them ; the specimens tested 

 must not, however, have remained long in the hand nor have been othei-wise warmed, neither 

 must they be too small. It is said to be possible for an expert to select, by the sense of 

 touch alone, a diamond out of a bag containing a large number of pieces of glass of similar 

 size and shape. 



Amber is one of the feeblest conductors of heat ; its conductivity is much less than that of 

 glass, hence a piece of amber can be easily distinguished from its imitation in yellow glass, 



E 



