Cabochon
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A cabochon or cabouchon is a gemstone which has been shaped and polished as opposed to facetted. The resulting form is usually a convex top with a flat back, although the term is used to mean any deliberate shape that is not facetted. Cutting "en cabochon" is usually applied to opaque gems, while facetting is usually applied to transparent stones. Hardness is also taken into account as softer gemstones with a hardness lower than 7 on the Mohs hardness scale are easily scratched, mainly by silicon dioxide in dust and grit. This would quickly make translucent gems unattractive, whereas polished as cabochons, the scratches would not be as apparent.
The one notable exception to this is the case of asteriated stones such as star sapphires and chatoyant stones such as cat's eye chrysoberyl, where a domed cabochon cut is used to show the star or eye, which would not be seen in a facetted cut.
The usual shape for cutting cabochons is an oval. This is because the eye is less sensitive to small asymmetricalities in an oval (as opposed to a round shape), and because the oval shape, combined with the dome, is attractive.
The normal procedure is to cut a slab of the rough rock, then to stencil a shape from a template. Normally, an aluminium scriber is used to mark the rock, as it tends not to rub off in handling. The slab is then trimmed to near the marked line, followed by nibbling to the line. The purpose of nibbling is to speed the shaping of the material. It arises because the saws are diamond tipped blades, whilst most grinding is done with silicon carbide grindwheels. The diamond cuts quicker and cleaner, and the saw does not wear as fast as the grind wheel.
Once the piece is trimmed and nibbled, it is usually mounted on a handle to assist manipulation. This procedure is called dopping: It is normally done by adhering the stone with wax onto a length of wooden dowel called a dop stick. The piece is then ground to the line, the edges are bevelled, and finally the top is smoothed to a uniform dome.
