Diamonds vary considerably in hardness, and even different parts of the same crystal differ in their resistance to cutting and grinding. Beautifully white diamonds have been found at Inverel, New South Wales, and from the rich yield of the mine and the white colour of the stones great things were expected. In the first parcel which came to England the stones were found to be so much harder than South African diamonds that it was at first feared they would be useless except for rock-boring purposes. The difficulty of cutting them disappeared with improved appliances, and they now are highly prized. The famous Koh-i-noor, when being 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, until, after working the mill for six hours at the usual speed of 2400 revolutions a minute, little impression was made. The speed was increased to more than 3000, when the work slowly proceeded. Other portions of the stone were found to be comparatively soft, and became harder as the outside was cut away. The intense hardness of the diamond can be illustrated by the following experiment. On the flattened apex of a conical block of steel place a diamond, and upon it bring down a second cone of steel. On forcing together the two steel cones by hydraulic pressure the stone is squeezed into the steel blocks without injuring it in the slightest degree. In an experiment I made at Kimberley the pressure gauge showed 60 atmospheres, and the piston being 3·2 inches diameter, the absolute pressure was 3·16 tons, equivalent on a diamond of 12 square mm. surface to 170 tons per square inch of diamond. The use of diamond in glass-cutting I need not dwell on. So hard is diamond in comparison to glass, that a suitable splinter of diamond will plane curls off a glass plate as a carpenter’s tool will plane shavings off a deal board. The illustration (Fig. 17) shows a few diamond-cut glass shavings. DENSITY OR SPECIFIC GRAVITY The specific gravity of the diamond varies ordinarily from 3·514 to 3·518. For comparison, I give in tabular form the specific gravities of the different varieties of carbon and of the minerals found on the sorting tables: SPECIFIC GRAVITY. Amorphous carbon 1·45-1·70 Hard gas coke 2·356 Hard graphite 2·5 Quartzite and granite 2·6 Beryl 2·7 Mica 2·8 Hornblende 3·0 Boart 3·47-3·49 Carbonado 3·50 Diamond 3·514-3·518 Garnet 3·7 Corundum 3·8 Zircon 4·4 Barytes 4·5 Chrome and titanic iron ore 4·7 Magnetite 5·0