82 GENERAL CHARACTERS OF PRECIOUS STONES 



in his hand and presses the stone on the grinding disc in the approximate position for each 

 facet. The rough form so obtained is then ground down more exactly as explained above. 

 With diamonds the preliminary shaping of the stone is performed by a process known as 

 bruting. This consists in rubbing together two diamonds, each being cemented at the end 

 of a stick or holder, until the desired form is obtained. The operation is performed over 

 a trough, so that the particles detached shall not be lost. 



In order to obtain the cabochon-cut or other rounded forms the dop or the stick of 

 cement is held in the hand and constantly turned so that the stone performs a rolling 

 motion on the surface of the rotating disc. The production of such rounded forms requires 

 a special skill. I'he harder and more quickly abrasive grinding powder will naturally be used 

 in these cases also and the stone afterwards finished with a suitable polishing material. 



The material of which the grinding disc or lap is made is varied according to the 

 hardness of the stone to be cut, harder metals being used for the harder stones and vice 

 versa. The disc may be made of iron, steel, copper, brass, tin, lead or pewter ; wooden 

 discs also ai-e sometimes used. The upper surface must be perfectly plane, but roughened 

 somewhat, at least near the maigin where the grinding takes place. The disc is usually 

 driven by water- or steam-power ; it rotates at the rate of two or three thousand revolutions 

 per minute, and sometimes at an even greater rate. The harder the stone to be cut the 

 greater is the rate at which the disc is rotated, as the rapid motion greatly intensifies the 

 action of the grinding powder ; so much is this the case that powder of the same substance 

 as the stone to be cut can be employed for grinding, provided the velocity of the disc is 

 sufficiently high. 



For the grinding of softer stones, especially varieties of quartz, a grindstone or disc of 

 sandstone is employed without grinding material other than the substance of the sandstone 

 itself. Other details of this process will be given later when we deal with the cutting of 

 agates. 



U'he material of the polishing disc or lap may be the same as that of the grinding 

 disc used for the same stone, but as a rule a softer material is employed. Polishing discs 

 are frequently made of wood covered with leather, cloth, felt or paper. 



The most important grinding or abrading material is corundum, the hardest of 

 all minerals except diamond. The pure and transparent varieties of this constitute the 

 ruby, sapphire, and other costly precious stones, but it occurs also in nature in an opaque 

 form in large masses ; and the finely granular, black variety known as emery is specially 

 abundant. This mineral owes its black colour to the inter-mixture of softer minerals which, 

 however, do not seriously affect its hardness. Emery occurs in large masses, especially in 

 Asia Minor and in the Island of Naxos in the Grecian Archipelago ; also at Chester in the 

 State of Massachusetts, U.S.A., and at other places. Commercial emery is largely obtained 

 from Naxos and from Asia Minor, and, in accordance with the various uses to which it is put, 

 is ground to various degrees of fineness. Crystallised common corundum is used for similar 

 purposes ; it occurs in a few localities in large masses of comparative purity, and not being 

 mixed with softer minerals possesses a hardness greater than that of emery. Other hard 

 minerals, such as topaz and garnet and sometimes even quartz, are occasionally employed as 

 grinding materials. 



In recent years an artificial grinding material, called carborundum., has been largely used 

 as an abrasive agent, especially in the United States. It was first prepared in large 

 quantities by the firm of Acheson of Pittsburg in Pennsylvania ; the method employed 

 being simply the fusing together of quartz-sand and coal at the enormously high temperature 

 of the electric furnace. The substance obtained is a compound of carbon and silicon 



