38 A Book of Precious Stones
	the blue ground minerals, adhered to grease, while all else would flow off with water as tailings. The improved shaking tables now used at the South African mines are corrugated, and while a first table fails to detain one third of the diamonds a second table recovers these, almost to the last diamond; so that this inven­tion is practically as certain in its accomplish­ments as the human eye and hand, while proving a great economy in its operation. It has been demonstrated also that these greased shaking tables will hold other precious stones of high specific gravity and hardness. The diamonds which are heavily coated with grease, of about the consistency of axle grease, by their experience with this process, are cleaned by boiling them in a solution of caustic soda. The quantity of deposit (diamonds) which reaches the sorting tables equals but one cubic foot in 192 cubic feet. From the sorting tables the diamonds are taken daily to the general office under an armed escort and delivered to the valuators in charge of the diamond department. These experts clean the diamonds of extraneous matter by boiling them in a mixture of nitric and hydro­chloric acids, or in fluoric acid. When cleaned