The first step in cutting is performing: roughly shaping the gem so that it has the profile of the gem shape we will be cutting. Choosing the shape and orientation of the final stone is an art. The cutter is looking to select the largest possible size and shape that the random shape of the rough will accommodate. Sometimes a crack or flaw in the crystal requires some creative thinking. It may be necessary to saw a piece of rough into two to get the best yield.

     Preforming is usually done by hand with fairly coarse diamond abrasive that cuts quickly. The idea in this step is to grind away any unneeded material quickly to save time later. Here is the same sapphire after performing: it’s roughly round and has a flat spot in the middle that will eventually be the table of the stone.

     Faceting machines vary, but they all have a rotating wheel that actually does the grinding, which is called the lap. The lap has a replaceable surface that is impregnated with bits of diamond. Diamond, while the most expensive of abrasives, cuts crisp, beautifully polished facets so quality cutters use it for cutting all gem varieties.

     The friction inherent in grinding and polishing a gem creates a lot of heat. In fact it can literally melt the surface of the facet. To keep things cool, the lap is lubricated by running water.

     In addition to a water-cooled spinning lap, all faceting machines have an arm that holds the gemstone to be cut on a stick called the dop. This arm must be able to be precisely positioned at a fixed angle and rotation to put each facet in the right place. This can be done as simply as using holed drilled into a board, which is called a jam-peg cutting machine. The Facetron machine we show here uses an index gear.