Coloured Gemstones: A Cut above Diamond

One of the most important considerations in dealing in coloured stones is cut. In fact, a good cut is more important for coloured stones than it is for diamonds, and often more difficult to achieve. Each species of coloured stones has different critical angles that will dictate the angles used in cutting. Dichroic and Trichroic stones need special attention to either display or hide the different colours. The secondary green colour in some blue Sapphires should be hidden while the interplay of blue and purple in Tanzanite or the bold purple and yellow of Ametrine are highly desirable.
With finer stones, especially in the Big Three of Sapphire, Ruby and Emerald cut should follow the classic shape for that species. For Sapphire and Ruby it is usually an oval shape and Emerald is usually cut in a rectangular step cut often called an emerald cut. The shapes should be well balanced and pleasing. Look for good colour and brilliance. The better and more expensive the stone, the more attention needs to be paid to cut. If you are knowledgeable about coloured gemstone cuts and the cutting process, a lot of money can be made by buying poorly cut stones at a discount and re-cutting for perfection.
Different cuts do matter
Other gemstones are usually cut to shapes that match the shape of the rough. Tourmaline is often cut in a very long step-cut or even a long oval while garnets are usually a more standard round or oval. Fine Imperial Topaz is typically cut in a long pear, oval or marquise to enhance the secondary red colour of the central axis of the crystal.
Stones that are more abundant in larger sizes of rough can be cut a variety of sizes shapes and styles. AmethystCitrine and Blue Topaz are found in all shapes, sometimes calibrated and sometimes wildly free-form. Free-form cutting usually takes its shape from what is left of the rough after inclusions and areas of poor colour are removed. In skilled hands these cuts can be spectacular, demanding equal skill from jewellery designer and stone-setters.
Coloured stones can be shaped in ways that are impossible with diamonds. Most coloured stones can accept curved facets. Patterns of concave facets on the pavilion of a brilliant cut stone can add tremendous life to the face-up appearance. Computer controlled faceting machines make these cuts available in commercial quantities. CNC carving mills make sculptural gemstones repeatable.
The bottom line for any coloured stone is to find cuts that are pleasing to the eye, lively, interesting and displays the beauty of the species to its best advantage. Although very high end stones should be cut to the standard shapes, don’t by-pass a different shape if it calls out to you. While there is a lot of science involved in cutting, the final judgement is more of an art form. Buy what attracts you and you can’t go wrong.

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