Gems are shaped entirely by being ground on abrasive wheels or revolving abrasive disks. For minerals that are no harder than quartz, natural sandstone wheels are sometimes used, but for the harder stones, such as rubies and sapphires, synthetic grinding wheels of cemented Carborundum (silicon carbide) must be employed.
The first step in the cutting of a gem is to saw it roughly to shape Thin abrasive disks or metal disks charged with powdered diamond or other abrasives are employed in this process. Wheels (called laps) made of Carborundum or of abrasive-charged cast iron are used to shape the stone. The stone to be shaped is cemented to the end of a wooden stick called a dop and is held against the revolving wheel or lap with the aid of a supporting block placed adjacent to the wheel. This supporting block contains a number of holes in which the end of the dop can be rested. By changing the dop from one hole to another the lapidary is able to control the angle of the facet, or face, being ground. When the stone has been ground to the required shape, it is brought to a high polish on wooden or cloth wheels charged with a fine abrasive such as rouge or tripoli powder.The oldest and simplest of the many standardized shapes or cuts given to gemstones is the cabochon cut, in which the stone is smoothly rounded. The cabochon cut is essential if a star or cat’s-eye is to be visible, and is the most satisfactory cut for opal, moonstone, and colorful opaque gems. Cabochon-cut stones usually are rounded on the back; this is sometimes advantageous in improving appearance, but often is done in order to give the stone extra weight.