Home » categories » rare book library » Catelle. The Diamond.

Ch. 6: Inherent Diamond Qualities & Cutting

Ch. 6: Inherent Diamond Qualities & Cutting Page of 448 Ch. 6: Inherent Diamond Qualities & Cutting Text size:minus plus Restore normal size   Mail page  Print this page
114
THE DIAMOND
as 7.5 to 7.8. That means that its hardness is half way or more between quartz and topaz, but the difference be­tween 7 and 8 is not nearly as great as between 9 and 10. It is said that the difference between 9 and 10 is greater than it is between 9 and 1. The scale therefore does not represent exact and absolute degrees of hardness, but is an arrangement of minerals of different degrees in that quality, numbered for convenient reference.
Nor is the diamond always of the same degree of hard­ness. Stones from wet diggings are usually harder than those from dry diggings. African diamonds are softer than Brazilians; Indian are harder, and those of Borneo and Australia are said to be hardest of all. The " nyf " or skin of a crystal is harder than the in­terior, and frequently there are knots in the grain, so much harder that it is difficult to cut them. Cut with or against the grain of a diamond, and the wheel makes little impression; it must be cut across the grain. Sir William Crookes is reported to have said of the Koh-i-noor, that in cutting one of the facets near a yellow flaw, the crystal became so much harder the further it was cut, that after working the mill for six hours at the usual speed, little impression was made, and that the work pro­ceeded very slowly even when the speed was increased to 3,000 revolutions per minute. Other portions of the stone were comparatively soft, but became harder as the outside was cut away.
The only rival of the diamond in hardness is the metal tantalum, of which it is said that in the effort to bore a hole through a plate of it, a diamond drill driven at the rate of 5,000 revolutions per minute for three days and nights, made a depression 1/4 mm. deep.
Ch. 6: Inherent Diamond Qualities & Cutting Page of 448 Ch. 6: Inherent Diamond Qualities & Cutting
Suggested Illustrations
Other Chapters you may find useful
bullet Tag
This Page