There are two main categories of faceted stones: round shapes and straight-sided shapes. The cutting of curved gemstone shapes has been dictated by the development of faceted diamond cuts, with the terminology and cutting methods also applied to coloured stones.
Usage and common faults
As the brilliant cut was originally designed for diamonds, it is typically used for pale, transparent material with a naturally high dispersion. Rose cuts were also designed for diamonds, but pyrites (marcasites) and garnets are frequently cut this way. Briolette and drops can be cut in most gemstone material, whether transparent, translucent or opaque, and can be drilled vertically or horizontally. The mixed cut is used regularly on corundum (ruby and sapphire), because it will improve the colour of pale stones.
The most common problem with round brilliant-cut gems is poor proportions. The cut may be dictated by the shape of the rough material, or the position of flaws and inclusions. In the Far East the aim of cutting is usually to retain as much weight as possible, whatever the proportions. A gemstone’s proportions will affect the intensity of colour; dark stones are sometimes cut shallow to brighten the stone, and pastel stones often need a deep cut for the best colour.
A brilliant cut or mixed cut will disguise irregular colour distribution much better than a step cut. If the colour is concentrated in the culet area of a deep pavilion, or a single plane of colour is run across the centre of the stone, the patchy colouring can appear perfect when viewed through the table. Flaws and inclusions can be successfully hidden under the upper girdle facets or around the outer edges of the pavilion facets, while the central area appears perfectly clean.
Careless cutting and poor proportions can be unsightly and leave a stone more vulnerable to breakage. Check that the girdle of a brilliant-cut stone is neither too thick (which will look ugly) nor too thin (which can make the gemstone fragile). Sharp culets are liable to damage and open culets can be seen through the table and will be reflected in the facets. An off-centre culet in a pale stone will be visible through the table, while darker colours may disguise lack of symmetry.
Weights and measures
If the pavilion is too shallow, light leaks through the bottom of the stone. The reflection will be dull and the gemstone will have a glassy “window” effect. Most small faceted gemstones are cut on machines that give exact calibration for their overall dimensions. The table at right provides the calibrated sizes of rounds with associated weights. As gemstones differ in specific gravity (density), the table gives the weights of a variety of gemstone types.