Cutting and Polishing Colored Stones




The operations required for fashioning cabochons include sawing, grinding or shaping, sanding, and polishing. If the rough material needs to be reduced in size, this initial step is usually accomplished with a diamond saw, a metal wheel charged with diamond dust. The "mud" saw, a metal which that utilizes silicon carbide as an abrasive, is seldom used. Thin diamond-charged bakelite or phosphor-bronze saws are often used for opal and other comparatively fragile materials. The shaping operation consists of grinding the sawed piece to the desired shape on silicon-carbide wheels of various grits under a constant stream of water, or on hard metal laps with silicon-carbide powder and water. Sanding is accomplished with hard wood wheels covered with coarse and fine-grit Carborundum paper. This operation, which usually requires the stone to be cemented to a dop stick, removes all scratches and does the final shaping. The stone is
then polished on hard felt, leather or cloth laps with tin oxide, cerium oxide, pumice, rouge or other polishing agents.

The faceting of transparent gem materials usually requires more planning and care than is necessary in fashioning opaque and translucent cabochon-cut stones. As motioned previously, proper orientation and attention to correct angles and proportions are important considerations. The roughing-out step is accomplished in much the same manner as with cabochon cuts. The table is usually roughed in first, after which a regular angle is formed around the crown and the girdle is set up. Then the pavilion is shaped in such a way that a fairly thick girdle is left to provide necessary material for placing and polishing the facets.

Faceting is accomplished with the jamb peg or with mechanical faceting heads. The jamb peg is an ingenious method of providing a rigid holder for the dop stick. Set perpendicularly to the horizontal lap is a board containing a number of indentations. The angle at which a facet is to be placed on a gemstone is controlled by the hole into which the pointed end of the dop stick is placed. By holding the end of the dop stick in one of the holes on the board to the lapidary can keep the cutting at a very definite angle, which can be duplicated on the other side of the stone.

A faceting head is a device that controls the angle of the dop stick to the lap and that is graduated so that the stone can be turned to polish any number of definite facets with the same angular relationship to the girdle. When the angle is set for one of the main-pavilion facets in a brilliant-cut stone, the faceting head can be left at the same angle while being turned through eight divisions in the 360° rotation. In other words 45°angles are marked on the holding device so that eight equally spaced facets can be placed. Most faceting devices can also be set to 16 fold symmetry.

The facets are usually cut on a horizontal cast-iron, copper or copper-alloy lap, using silicon carbide, boron carbide or diamond dust as the abrasive. After the crown facets are cut, they are usually polished while the stone is still cemented to the dop stick in its original position. A number of different polishing laps are used, including lead, tin, pewter, wood and pitch. Tin oxide, tripoli, pumice, rouge, chromic oxide, alumina, titanium oxide and cerium oxide are utilized for the polishing agents.