The May gemstone, the emerald, is beautiful. It can range in colour from a pure dark green to a bluish green to a softer forest green, with some yellow. All are ‘green’ but the shades of green are breathtaking.

Apart from the colour, there are two things to note about emeralds. The first is that while they are a hard stone, they can also be quite brittle with a tendency to crack along fracture lines and to chip when worn. In the 1400s and 1500s, a table cut for gemstones was developed, with a flat upper surface, square or rectangular, with the lower part of the stone, the pavilion, coming to a point or a flat surface. The emerald cut for gemstones evolved from this cut as it helped to reduce the risk of emeralds cracking when being cut or set as the shape followed the structure of the gem. As well as the flat table and long pavilion, the cut has stepped facets along the sides. The cut also protects the corners of the stone from chipping. The emerald cut was first named as such in the 1920s, and the cut requirements were standardised in the 1950s.

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Antique emerald and diamond navette ring

Of course, the emerald cut is not the only cut used for emeralds. The ones in the navette ring above are a rose cut, for instance. Often, emeralds can be cut en cabochon, particularly if heavily included like the ones in the bottom photo.

This leads to the second item of note about emeralds, which is the fact that they are rarely completely clear. They often have a number of inclusions and fractures. The presence of these inclusions can be a good sign that the gemstone is natural and not synthetic. Because of these inclusions, most emeralds will have been treated in some way, but some treatments are more acceptable than others. Heating and oiling of emeralds using a colourless oil like cedarwood oil or Canadian balsam oil is an accepted treatment and has a long history in the industry. It is used to fill internal fissures inside emeralds which improves the colour. The treatment is a stable one but not permanent and should be disclosed, however, it is often considered to be industry practice and so well known that disclosure should not be necessary. Some emeralds are treated with coloured oils or resins such as opticon. The use of coloured oils must be disclosed to purchasers. There are other treatments but oiling and filling are the main ones used.

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Antique Roman or early medieval emerald earrings

Despite the problems with inclusions and internal fractures, there is just something about emeralds that pulls you in and makes you want one.