10.08.2023 Surface treatment of gemstones: from crack filling to lasering
Most gemstones are treated in some way – with the aim of improving them. There are many different methods for surface treatment of gemstones, using oil, resin, glass and other substances.
Gemstone treatments: Surface texture refinement
Untreated gemstones are particularly valuable and best suited as an investment. However, there are not enough gemstones that are naturally of pristine quality, so most are treated. This may seem sensible for aesthetic reasons, but treatments reduce the value of the minerals. This excludes cutting and polishing, which are what turn a rough stone into a sparkling gem in the first place.
Typical treatments are firing or heating and irradiation. There are also surface treatment methods for gemstones. Some are permanent – such as heating – while others are temporary and can be reversed. Oil and resin, for example, used for crack filling and impregnation, dry out and dissolve over time.
Cracks in gemstones are filled with colorless substances such as oil, resin, glass or plastics to conceal the cavities and make them unrecognizable to the naked eye. Usually, the fillers are fused to the original stone by heating.
Ruby: Crack filling with lead glass
In ruby, lead glass is used as a filler, and the liquid glass is forced into the cracks and bonds with the gem substance as it solidifies. The treatment is not permanent, as lead glass is very sensitive to acid. Even lemon juice is enough to damage the glass filling.
Emerald: crack filling with cedar oil or resin
Due to its crystalline structure, the emerald often has inclusions and cracks, which is why the demands on its purity are less strict and these blemishes are rather considered beauty spots. Hardly any emerald in the trade is not treated.
In the case of emerald, cedar oil is used to improve its purity. The refraction of the oil is similar to that of the gemstone, which is why the cracks virtually disappear to the human eye. During the treatment, the stone is first placed in acids to clean it, and then in a heated hydraulic cylinder with cedar oil. Sometimes a vacuum pump is also used to pull the air out of the cracks so that the oil can penetrate deeper. Nowadays, resins are also often used to fill cracks in emeralds – though this practice is controversial in the industry.
Diffusion treatment is a high-temperature treatment involving the addition of color-producing elements (e.g., chromium or beryllium), which are introduced into the surface layer of the gemstone. This enhances the intensity of the color – however, it can happen that the artificially created color disappears again through polishing or grinding.
Vapor deposition or coating
Gemstones can be artificially colored by means of physical vapor deposition (PVD). The surface of the stone is coated with an extremely thin layer of metal (e.g. titanium or gold), which then hardens and increases the color intensity. This is how the rainbow colors of the mystic topaz are created, for example.
To stabilize gemstones and make them more resistant, as well as to conceal minor cracks, porous and fragile specimens are sometimes impregnated with synthetic resin, or more rarely, wax or kerosene. In this process, the cracks are first filled with resin and then sealed with UV light. This treatment additionally smoothes the surface of the stones.
Laser treatment has been used for diamonds for not too long. Laser drilling involves drilling into inclusions, dissolving them with acid, and finally filling them with a transparent substance such as glass or synthetic resin.
Treatments must be declared
Treatments must be transparently declared when purchasing a gemstone. In order to improve consumer protection, the International Confederation of Jewelry, Silverware, Diamonds, Pearls and Gemstones (CIBJO) has formulated globally applicable rules for the trade, which also address the treatment of gemstones.
Apart from cutting and polishing, all treatments must be clearly declared, as they have an influence on the value of the gemstone. A certificate from an internationally recognized, independent gemmological institute is therefore a must when buying gemstones.