138 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



Whether diamonds are capable, as a rule, of giving out light after being raised to a 

 temperature below red-heat is doubtful, but some specimens, which are unaffected by 

 sunlight and remain quite dark, are induced to glow by exposure to electric sparks. But 

 here, as in all cases, the phosphorescence can be produced only when the stone has not 

 previously been exposed to a strong heat. 



The light given out by a phosphorescing diamond is in almost all cases feeble, and 

 much less intense than that emitted by many other phosphorescent substances. The light, 

 which is usually yellow in colour, but may under certain circumstances be blue, green, or 

 red, is strongest when induced by electric sparking. Remarkable observations have been 

 made on the appearance of different faces of a phosphorescing crystal of diamond. 

 Dessaignes (1809) stated that a diamond after exposure to the sun's rays emitted light 

 from the cube faces, but not from the octahedral faces, which remained quite dark. 

 Maskelyne described a diamond crystal which emitted a beautiful apricot-coloured light 

 from the cube faces, a bright yellow from the faces of the rhombic dodecahedron, and a 

 yellow light of another shade from the octahedral faces. 



All these appearances are, as a rule, of brief duration. A case is, however, recorded of 

 a diamond which continued to phosphoresce for an hour after the removal of the exciting 

 cause. 



The phosphorescence of diamond is said to have been first observed in 1663 by the 

 famous English physicist Robert Boyle. 



7. ELECTRICAL AND THERMAL CHARACTERS. 



A diamond, whether rough or cut, becomes positively electrified on rubbing; the 

 charge so acquired is quickly lost, never being retained more than half an hour. In 

 contrast to graphite, the other crystallised modification of carbon, which is a good conductor 

 of electricity, the conductivity of diamond is so inappreciable that the stone ranks as a non- 

 conductor. 



Diamond being a good conductor of heat appears cold to the touch, and by this means 

 can be distinguished from other substances, as already explained. 



B. OCCURRENCE AND DISTRIBUTION OF DIAMOND. 



Diamond has been found in all five continents, but not to the same extent in each. It 

 has been longest known in Asia, where the famous old Indian deposits have probably been 

 known and workeil from the earliest times; now, however, they are almost completely 

 exhausted. In close geographical connection with these are the deposits in Borneo, but 

 the supply from this island, in comparison with the rich treasure of India, has always been 

 limited. Reported discoveries of diamonds in the Malay Peninsula, where, according to one 

 account, the famous " Regent" of the French crown jewels was found, in Pegu and Siani 

 and the islands of Java, Sumatra and Celebes, are for the most part unauthenticated ; and 

 the same may be said of the reported occurrence in China (province Shan-tung), 

 Arabia, &c. 



In America the famous Brazilian diamond-fields were discovered at the beginning of 

 the eighteenth century, and have compensated for the exhaustion of the Indian mines ; the 

 richest yield of stones has been given by the mines in the States of Minas Geraes and 

 Bahia. Recent finds have been made in another part of the South American continent, 

 namely in British Guiana. Well authenticated, but of little commercial importance, is the 

 occurrence of diamond in the United States of North America ; a small number of stones 



