22 



TUESDAY, DECEMBER 6th. 



A lecture was delivered by F. W. Rudler, Esq., F.G.S., on 

 "Diamonds, Natural and ArtificiaL" 



After a brief notice of the history of the diamond, with special 

 reference to its mention by PUny and other Roman wTiters, the 

 Lecturer addressed himself to the study of its physical characters, 

 commencing with its crystalline form. By aid of diagrams and 

 illustrations on the blackboard he explained the principal features 

 of its crystaUization. All the forms of the diamond may be 

 referred to the cubic system. It was pointed out that the crystals 

 are often " twinned," and the faces strongly curved. However 

 diverse the external forms of the diamond may be, it invariably 

 admits of being split, or "cleaved," in directions parallol to the 

 faces of a regular octahedron. Advantage is taken of this cleavage 

 in trimming a rough diamond, so as to remove flaws and to reduce 

 an irregularly shaped stone to sj-mmetrical proportions. 



The old popular notion that a diamond, if placed on an 

 anvil and struck by a hammer will resist the blow, arose from con- 

 founding the property of hardness with that of toughness. Although 

 the diamond is so brittle as to be broken by a gentle tap, it is yet 

 so hard as to resist abrasion by any natural substance. Its supreme 

 hardness has led to its use by the lapidary, by the glazier, and by 

 the engraver. The diamonds which are mounted in the steel crown 

 of the " Diamond Rock-drills " are not crystals, or crystalline 

 splinters, but merely fragments of the black imcrj^stallizable variety, 

 known in Brazil as Carbonado. 



The Lecturer described the various steps in cutting and polish- 

 ing a diamond, and referred to the recent revival of the trade in 

 Clerkenwell. The tlistinction between "brilliants" and "roses" 

 was pointed out. Wlien cut as a well-proportioned brilliant, and 

 polished, the diamond displays to the greatest advantage its extra- 

 ordinary brilliancy of lustre; while its high dispersive power 

 contributes largely to its beauty. Although the purest diamonds 

 are colourless, the stone frequently exhibits slight tints, and in 

 some cases pronounced colours. Diagrams and models of the blue 



