84 GENERAL CHARACTERS OF PRECIOUS STONES 



Garnets of fairly considerable size for use as larger ornaments are ground with emery 

 or garnet-powder on a leaden disc and polished on a tin disc with tripolite and sulphuric 

 acid. Smaller garnets, on the contrary, which are strung as beads after being pierced with a 

 fine diamond point, are ground on a disc of fine sandstone with emery and olive-oil, and 

 polished on a wooden disc with tripolite and water or on a tin disc with tripolite and 

 sulphuric acid. Rock-crystal and amethyst are ground on a copper or lead disc with emery, 

 and polished on a tin disc or on a wooden disc covered with felt, with putty-powder, 

 tripolite or bole. For grinding agate, jasper, chalcedony, carnelian, and chrysoprase a disc 

 of copper, tin or lead is often used with garnet or topaz-powder instead of emery ; while 

 for polishing, pumice-stone, jeweller's rouge or putty-powder on a tin disc, or pumice-stone 

 on a wooden disc is employed. Agate and other varieties of quartz are, however, often 

 worked in another manner, which will be considered under the special description of agate. 



d. Soft stones. Obsidian, chrysolite, opal, adularia, turquoise, lapis-lazuli. 

 Ground with emery on a disc of lead or tin. 



Polished with tripolite on tin or hard wood, or sometimes with pumice-stone on wood. 



e. Glass imitations. These are usually ground and polished on wooden discs ; 

 emery being used as the grinding material, and tripolite as the polishing material. 



Not infrequently, before the process of grinding a precious stone can proceed, a 

 preliminary preparation of the stone is required. Many precious stones, especially the most 

 valuable, such as diamond and ruby, occur in nature in relatively small but perfectly pure 

 fragments. In such cases no preliminary preparation is needed, and the stone can be at 

 once cut into any desired form. It is otherwise, however, with many precious stones, such, 

 for example, as aquamarine ; the naturally occurring crystals and fragments of such stones 

 may be too large for a single cut stone, or may include material which is cloudy or faulty 

 and unfit for cutting. In such cases the specimen must be divided into several pieces of 

 convenient size, and the faulty and undesirable portions removed by a process less lengthy 

 and costly than that of simply grinding them away. 



This operation is performed with the aid of a thin metal disc, usually of soft iron, the 

 edge of which is charged with some hard cutting powder, preferably diamond. This cutting 

 or slitting disc usually rotates in a vertical plane on a horizontal axis ; by pressing the stone 

 against the cutting edge of the rapidly rotating disc it will be slowly cut through by the 

 small splinters of diamond which are embedded in the metal. The division of the stone can 

 also be effected by means of a wire smeared with cutting powder, stretched in a bow and 

 used as a saw. This method is, however, very slow, and is now rarely practised. 



The superfluous or undesirable material of cheap stones, which occur in large masses, is 

 not cut away, but simply broken off by the blow of a hammer ; a mode of procedure which 

 cannot be recommended for valuable stones, unless, as in diamond and topaz, they possess 

 certain known directions of easy cleavage. Stones which can be cleaved with such facility 

 may, as we have already seen, be readily reduced in certain directions with a chisel and 

 hammer with no fear of losing valuable material, so that by this means the work of cutting 

 and grinding is considerably lessened. This subject will be again considered, especially 

 with reference to the diamond, under the special description of each stone. The fragments 

 of valuable material thus broken or cut away are carefully collected and preserved ; they 

 may be utilised for the fashioning of smaller gems, or, if of sufficient hardness, pulverised 

 and used as a cutting and grinding material. 



As the treatment of precious stones must be varied to suit the different nature of each 

 stone, each requiring different contrivances and methods, so different branches of the gem- 

 cutting industry can be identified, each establishment dealing, as a rule, with but one kind 



