80 GENERAL CHARACTERS OF PRECIOUS STONES 



A notice of all the technical details connected with the cutting of precious stones would 

 be entirely out of place here. Only the main principles of the methods which are applicable 

 to all precious stones will here be considered. The special modifications of general methods 

 which are necessary for certain stones, particularly diamond, will be considered when we 

 come to the special description of such stones. 



In the process of grinding, the harder stone or cutting material, by means of which 

 the rough stone is fashioned into a gem, is almost invariably used in the form of a fine 

 powder. This grinding powder is mixed to a paste with olive-oil (in the case of diamond 

 powder) or with water (in the case of emery, &c.), and placed near the edge of a circular 

 disc or lap about a foot in diameter and an inch in thickness. This disc, which is usually 

 of metal, revolves with great velocity in a horizontal plane about a vertical axis. The 

 precious stone to be ground down is pressed against that part of the disc on which the 

 grinding powder lies. The pressure of the stone against the revolving disc causes the 

 powder to become embedded in the soft metal of the disc. This then acts as a file, the 

 hardness of which is equal to the hardness of the grinding powder. By the gradual 

 abrasion of the material of the stone over the area which is being ground the facet 

 develops, the length of time occupied depending on the hardness of the precious stone and 

 of the grinding material. 



At short intervals during the progress of the work the gem-cutter must ascertain the 

 size to which the facet has grown, so as to avoid making it too large. A facet which 

 exceeds the other corresponding facets in size is said to be over-ground. Such an irregularity 

 in a stone greatly diminishes its value. Great care is also necessary to avoid the over- 

 heating of the stone during the operation of grinding. Neglect of this precaution results in 

 the development of small cracks, known as " icy flakes," in the interior of the stone, which 

 cause it to be dull and so diminish its beauty. Before beginning the actual work of 

 grinding, the particular form in which the stone is to be cut, and also its most favourable 

 orientation with respect to the boundaries of the rough stone, must be decided so that the 

 precise position and direction required for each facet may be given to it by the cutter. 



One of the most obvious essentials in the grinding of a stone is a means whereby it may 

 be retained constantly in one position. For this purpose a special kind of holder, the so- 

 called dop (or dopp) is used. This consists of a small hemispherical cup of copper attached 

 bv the convex side to a stout copper rod. The cup is filled with an easily fusible alloy of tin 

 and lead, which is fused and allowed to cool ; immediately before solidification sets in, the 

 stone is placed in the cooling alloy in the position desired, and so that about one-half is 

 embedded in the alloy and the other half projects out of it. By this means the stone is 

 fixed in the holder in an unalterable position. In the case of stones of comparatively little 

 value the holder just described may be replaced by a stick of cement, or by a rod of metal 

 or wood, to which the stone may be fixed by a cement consisting of pitch, resin or shellac, and 

 the finest brick-dust. 



The rod of the dop or the stick of cement is fixed in a clamp at one end of a small bar, 

 perpendicular to which at the other end are two short legs. This apparatus is placed so 

 that the two legs rest on a fixed table, while the precious stone in its holder rests upon the 

 grinding disc, which is parallel to and a little higher than the table. The stone is 

 pressed against the upper surface of the disc by loading the bar with leaden weights, 

 which are larger or smaller according to the hardness of the stone to be ground. To 

 avoid irregularly loading the grinding disc, two stones are ground at the same time, these 

 being placed at opposite ends of a diameter of the disc. In the case of a stone of little 

 value the rod to which it is fixed is simply held in the hand until the facet is ground down 



