GEOLOGY. 303 



gold than has yet been brought, according to the statistics, from Cali- 

 fornia and Australia. 



It is also apparent that every time a cartload of clay is hauled out 

 of a cellar, enough gold goes with it to pay for the carting. And if 

 the bricks which front our houses could have brought to their surface, 

 in the form of gold leaf, the amount of gold which they contain, we 

 should have the glittering show of two square inches on every brick. 

 Am. Phil. Society Proc. 



CLEAVAGE OF THE DIAMOND. 



Although the diamond is so hard, it is very easily broken ; and, 

 indeed, by a particular knack, it may even be cut with a common 

 penknife. This apparent anomaly is due to what is called its cleav- 

 age, a result of the crystalline structure. Many well-known sub- 

 stances as slate, for example split or cleave with peculiar facility 

 in certain definite directions, while they offer considerable resistance 

 to fracture in all others. The diamond has tin's property, cleaving 

 easily in no less than four directions, parallel to the surfaces of the 

 octohedric crystal ; and, therefore, when moderate force is applied in 

 either of these ways, the stone splits into pieces. Pliny, mentioning 

 the great hardness of the diamond, states that if laid upon an anvil, 

 and struck with a hammer, the steel would sooner give way than the 

 stone. This assertion is a matter of popular belief in the present 

 day, but we would not recommend any possessor of a good diamond 

 to try the experiment. The chances of some of the forces acting in 

 the cleavage directions are so great, that the stone would in all proba- 

 bility fly to pieces under the first blow. The truth is, that Pliny 

 referred not to the diamond, but to the sapphire, which, though less 

 hard than the diamond, cleaves only in one direction, and ni'ght, 

 therefore, withstand the test named. The cleaving property of the 

 diamond is made useful in two ways in the manufacture : first, by 

 splitting the stones when they contain flaws, and, secondly, in the 

 preparation of diamond powder. When a rough diamond is seen to 

 contain a defect of sufficient extent to depreciate its value as a single 

 gem, it is split in two, precisely at the flaw, so as to make two sound 

 stones. This is a very simple operation in appearance, done in a few 

 seconds, but it requires an amazing amount of skill to do it properly. 

 The workman, by a sort of intuitive knowledge, gained by long expe- 

 rience, knows, on a careful inspection of the stone, the exact direc- 

 tion which a cleavage plane passing through the flaw will take. 

 Tracing this plane, therefore, to the exterior, he makes on the edge 

 of the stone, precisely in the same spot, a slight nick with another 

 diamond. He then places a small knife in that nick, gives it a light 

 tap with a hammer, and the stone at once cleaves in two, directly 

 through the flaw. This operation, in daily practice in the Amster- 

 dam diamond works, is one of the most elegant and instructive pro- 

 cesses in the whole range of mineralogy. It is reported that Dr. 

 Wollaston, celebrated as almost the originator of the science of crys- 

 tallography, once made a handsome sum by purchasing a large flawed 

 diamond, at a low price, and subsequently splitting it into smaller 



