Facet cutting
Facets are flat faces on geometric shapes. Gemstones commonly have facets cut into them as a way to improve their appearance.
Of the many hundreds of facet arrangements that have been used, the most famous is probably the round brilliant cut, used for diamond. This arrangement of 57 facets was calculated by Marcel Tolkowsky 1919. It has been tweaked and improved a little since then, normally including a 58th facet (a culet) on the bottom of the cut. Since this is calculated to show maximum brilliance diamonds are rarely cut in any other arrangement, although recently the Princess cut is becoming popular.
Cutting facets
The art of cutting a gem with facets is a very precise activity. The aim with a facetted cut is to produce an article that sparkles with internally reflected light, and that shows of the fire of the stone. Accordinally normally only transparent or translucent are faceted.
The angles of each facet is precisly calculated. As the aim is to maximise the effect of the internal reflections, these angles depend on the refractive index of the material. This means that although the name and general shape of a particular cut may be the same between different materials, the actual angles will be slightly different, for the maximum effect.
Thus, although diamond, cubic zirconia and rock crystal may all look the same, and can be cut in a round brilliant cut, the angles must be different to get the maximum effect. Additionally, as diamond has a refractive index significantly higher than the other transparent stones, it can have a much greater sparkle than other materials.
Specialised machines are used for cutting faceted stones. These consist of two main features: a flat abrasive, usually diamond dust of precise size bonded onto cloth; and a system for holding a stone onto the pad that measure the position of the stone. This usual requires the stone to be attached to a holder, which is then placed in an indexed vice. This allows the abrasive to be moved to fined stages, and the stone replaced to exaclty the same angle as before. Modern machines tend to have indexed gears for moving the stone, so that moving the stone to cut the next facet is a precise operation.
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