DIAMONDS. 223 



In the substatfce of many diamonds we find inclosed black uncrys- 

 tallized particles of graphite. There also occur what may be considered 

 intermediate forms between the well-crystallized diamond and graphite. 

 These are "bort" and "carbonado." Bort is an imperfectly crystal- 

 lized diamond, having no clear portions ; therefore it is useless for gems. 

 Bort is frequently found in spherical globules, and may be of all colors. 

 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 crystallized form of carbon. It 

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

 pebbles, sometimes weighing two or more ounces. 



Diamonds vary considerable 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 farther 

 it was cut into, until, after working the mill for six hours at the usual 

 speed of 2,400 revolutions a minute, little impression was made. The 

 speed was accordingly increased to more than 3,000, when the work 

 slowly proceeded. Other portions of the stone were found to be com- 

 paratively soft, and became harder as the outside was cut away. 



Beautifully white diamonds have been found at Inverel, New South 

 Wales, and from the rich yield of the mine and the white color of the 

 stones great things were expected. A parcel of many hundred carats 

 came to England, when it was found they were so hard as to be prac- 

 tically unworkable as gems, and I believe they were ultimately sold for 

 rock-boring purposes. 



I will illustrate the intense hardness of the diamond by an experi- 

 ment: I place a diamond on the flattened apex of a conical block of 

 steel, and on the diamond I bring down a second cone of steel. With 

 the electric lantern I will project an image of the diamond and steel 

 faces on the screen, and force them together by hydraulic power. 

 Unless I happen to have selected a diamond with a flaw, I shall squeeze 

 the stone right into the steel blocks without injuring it in the slightest 

 degree. 



But it is not the hardness of the diamond so much as its optical 

 qualities that make it so highly prized. It is one of the most refract- 

 ing substances in nature, and it also has the highest reflecting proper- 

 ties. In the cutting of diamonds advantage is taken of these qualities. 

 When cut as a brilliant the facets on the lower side are inclined so 

 that light falls on them at an angle of 24° 13', at which angle all the 

 incident light is totally reflected. A well-cut diamond should appear 

 opaque by transmitted light, except at a small spot in the middle where 

 the table and culet are opposite. All the light falling on the front of 

 the stone is reflected from the facets, and the light passing into the 

 diamond is reflected from the interior surfaces and refracted into colors 



