488 Mr. William Croohes [June 11, 



crystals (macles) are not uncommon. Having no double refraction 

 it should not act on polarised light. But, as is well known, if a 

 transparent body which does not so act is submitted to strain of an 

 irregular character it becomes doubly refracting, and in the polari- 

 scope reveals the existence of the strain by brilliant colours arranged 

 in a more or less defined pattern according to the state of tension 

 in which the crystal exists. Under polarised light I have examined 

 many hundred diamond crystals, and with few exceptions all show 

 the presence of internal tension. On rotating the polariser, the 

 black cross, which is most frequently seen, revolves round a par- 

 ticular point in the inside of the crystal, and on examining this 

 point with a high power, we see sometimes a slight flaw, more rarely 

 a minute cavity. The cavity is filled with gas at an enormous 

 pressure, and the strain is set up in the stone by the effort of the gas 

 to escape. 



It is not uncommon for a diamond to explode soon after it 

 reaches the surface, and some have been known to burst in the 

 pockets of the miners or when held in the w^arm hand. Large 

 crystals are more liable to burst than smaller pieces. Valuable 

 stones have been destroyed in this way, and it is whispered that 

 cunning dealers are not averse to allowing responsible clients to 

 handle or carry in their warm pockets large crystals fresh from 

 the mine. By way of safeguard against explosion, some dealers 

 imbed large diamonds in raw potato to insure safe transit to 

 England. 



I will project some diamonds on the screen by means of the 

 polarising microscope, and you will see by the colours how great 

 is the strain to which some of them are exposed. 



In the substance of many diamonds we fiind enclosed black 

 uncrystallised particles of graphite. There also occur what may 

 be considered intermediate forms between the well-crystallised 

 diamond and graphite. These are "boart" and "carbonado." 

 Boart is an imperfectly crystallised diamond, having no clear por- 

 tions, and therefore useless for gems. Boart is frequently found 

 in spherical globules, and may be of all colours. It is so hard that 

 it is used in rock-drilling, and when crushed it is employed for 

 cutting and polishing other stones. Carbonado is the Brazilian 

 term for a still less perfectly crystallised form of carbon. It is 

 equally hard, and occurs in porous masses, and in massive black 

 pebbles, sometimes weighing a couple or more ounces. 



Diamonds vary considerably in hardness, and even different parts 

 of the same crystal are decidedly different in their resistance to 

 cutting and grinding. The famous Koh-i-noor, when cut into its 

 present form, showed a notable variation in hardness. In cutting 

 one of the facets near a yellow flaw, the crystal became harder and 

 harder the further it was cut into, until, after working the mill for 

 six hours at the usual speed of 2400 revolutions a minute, little 

 impression was made. The speed was accordingly increased to more 



