DIAMOND: PHOSPHORESCENCE 137 



page of one of his books as " Inventeur du precede de decoloration du diamant." Our 

 present knowledge of the constituents of diamond pigments makes us unwilling to believe too 

 readily in the possibility of their complete destruction ; at all events, no method is at present 

 known which is effectual in all cases. 



Although it is impossible actually to destroy the colour of yellow diamonds, such as 

 now so often come into the market, it is easy to disguise their yellow tint and make them 

 appear colourless. This device has often been practised for fraudulent purposes, and was 

 successfully carried out in Paris a few years ago. The yellow stone is placed in a violet 

 liquid, such as the dilute solution of potassium permanganate used for disinfecting purposes. 

 On being taken out and allowed to dry the stone becomes coated with a thin film of the violet 

 substance. The combined effect of yellow and violet colours in certain proportions is to 

 produce in the eye the sensation of white light ; hence, when the violet layer is of a certain 

 thickness, the stone will have the appearance of being perfectly colourless. Should it still 

 appear yellow after the first trial, it may be immersed a second or third time, while if the 

 violet colour is too deep, some of it may be washed off. Instead of a solution of 

 potassium permanganate violet ink may be used for this purpose. A stone treated in this 

 way will, of course, remain apparently colourless only so long as the violet coating remains 

 intact ; directly this is rubbed off, which readily happens, the yellow colour of the stone 

 becomes apparent. A more permanent coating is said, however, to have been recently 

 devised for this purpose. The methods by which yellow stones can be made to appear 

 colourless are very old; ultramarine is supposed to have been used by the ancient 

 inhabitants of India for this purpose. 



According to the investigations of Petzholdt, which are confirmed by every diamond- 

 cutter, the colour of diamond powder or dust varies from gi-ey to black, and is darker the 

 finer its state of division. 



Phosphorescence. — Many en-oneous statements have been made with regard to the 

 phosphorescence of diamond. Thus it has been stated that diamond phosphoresces in 

 darkness after exposure to the direct rays of the sun, the phosphorescence being specially 

 marked after exposure to blue light, and less so after exposure to red light. It is even said 

 that after being screened by a board or paper from the direct rays of the sun, so that the 

 stone is exposed to diffused daylight only, it will be seen to glow brightly when placed in 

 darkness. Exact researches, however, have proved that only very few diamonds phospho- 

 resce after exposure to sunlight, and that neither the direct rays of the sun nor intense 

 artificial light cause phosphorescence in the majority of stones. Streeter reports that a 

 yellow stone of 115 carats, after exposure to lime-light lit up a dark room ; and Edwards 

 describes a water-clear diamond of 92 carats, which after one hour s exposure to electric 

 light emitted in a dark room a light which lasted for twenty minutes, and was so strong 

 that a sheet of white paper placed near the stone could be distinctly seen. Of 1-50 

 diamonds of various forms, sizes and qualities examined by Kunz, only three showed the 

 phenomenon of phosphorescence after exposure to the light of the electric arc. 



Although exposure to light has in diamond so small an effect in exciting phospho- 

 rescence, the phenomenon is easily produced by rubbing the stone. Kunz observed that all 

 the diamonds he examined, without exception, became self-luminous after being rubbed on 

 wood, leather, woollen or other material. Some stones needed only to be drawn once 

 across the substance, especially if it were of wool, to render them self-luminous ; the most 

 marked phosphorescence was, however, developed in stones by rubbing on wood against 

 the grain. According to other statements, rubbing on metals (iron, steel, copper) is 

 effective. 



