Optical properties < 



ARTIFICIAL, PRECIOUS STONES — HEATON. 219 



and also of deciding whether an alleged gem is genuine or imitation 

 with ease and certainty. 



The scientific examination and identification of gems in this 

 manner is a matter of the greatest interest, but it would take far too 

 much time to discuss it in detail, and it is quite unnecessary to do 

 so, because it has already been brought before the society most 

 exhaustively by our chairman, Dr. Miers. 1 I propose, therefore, 

 merely to remind you of the main points by means of the accom- 

 panying summary (Table I). 



Table I. — Properties influencing the value of precious stones and used as means of 



identification. 



fColor. 



(Cleavage. 

 Lamination. 

 Inclusions. 



Refractive power [refractometer]. 



Double refraction [polariscope]. 



Pleochroism [dichroscope]. 



Dispersion. 



Absorption spectrum [spectroscope]. 



(Hardness [hardness points]. 

 Toughness. 

 Chemical composition. 

 (Specific gravity. 

 Thermal conductivity. 

 X rays. 



In order to bring this matter up to date in the records of the 

 society, however, I must refer briefly to one or two particulars in 

 which advance has been made since the time of these lectures. 



The most important properties of a precious stone are those 

 depending upon its refractive powers. Until recently the accurate 

 determination of the refractive index of a stone was a matter involv- 

 ing the use of complicated and expensive instruments, and a matter for 

 the skilled mineralogist rather than the practical jeweler. It is 

 true that at the time Dr. Miers published his lectures there existed 

 an instrument known as the reflectometer, but the determination 

 of the refractive index with this was a matter of some difficulty 

 even in skilled hands, and its value for commercial purposes was 

 very small. Since that time, however, thanks to the ingenuity 

 of Dr. Herbert Smith, this instrument has been improved out of all 

 recognition, and in its place we have the Herbert Smith refractom- 

 eter (pi. 1, fig. 1), by means of which anyone of normal common 

 sense can determine the refractive index of a stone in a few seconds 

 without even removing it from its setting, and which, with a little 



i Cantor Lectures on Precious Stones, April, 1896. 



