– altering a gem’s appearance by applying a coloring agent like paint to the back surfaces of gems (a treatment known as “backing”), or paint applied as a coating to all or a portion of a gemstone’s surface with the effect of altering the color.
1. The most commonly encountered coated gems include: Diamonds – Thin-film coatings are sometimes used on diamonds to change their color. Crude, yet effective coatings can also include the use of permanent ink markers along the girdle surface of a diamond, causing its face–up appearance to be affected by the color of the ink used. More modern coating methods use metal oxide thin films.
The intense pink color of these three diamonds is the result of a surface coating.
Tanzanite – Though rarely used, tanzanites have been coated to improve the intensity of their blue-violet color.
Classic tanzanite exhibits a deep, purplish blue color (left). Pale tanzanite, and other pale stone species are sometimes coated with an ink-like substance to try to deepen and improve their color (right).
Topaz – Some colorless topaz is coated with metal oxides to create the appearance of a variety of different colors. In the past, such treatments were often described as a form of “diffusion” of a chemical into the surface of the gemstone, but this was a misnomer since in most cases the added color was confined to the surface of the gemstone.
Some natural topazes are colorless (top two), but they can be coated with metal oxides to produce a variety of metallic colors (bottom).
Coral – Some black coral (also known as Horn coral) has been reported as bleached and then coated with relatively thick layers of artificial resin with the goal of protecting the coral and intensifying its color.
This golden coral is the result of a two-step process: one to bleach away the dark color (the coral branch has been partially dipped in bleach to result in the golden color). The coral is then coated with a resin to deepen the tone and protect the coral.
Pearls – Some pearls reportedly have been treated with a colorless hard coating in an effort to improve durability.
Quartz – Occasionally, quartz is coated with metal oxides to create colors rarely seen in natural quartz.
Vapor deposition can coat many types of gems with metal oxides. This thin layer can alter the color of whatever it coats, such as the quartz crystals, or already faceted quartzes such as those shown here.
2. Durability factors – Because they tend to be softer than or may not adhere well to the underlying gem, thin-film surface coatings of any kind are susceptible to scratching, particularly along facet edges and junctions. Care should be taken to not allow any hard or abrasive objects to come in contact with coated gems.
3. Detectability - Once suspected, the treatment is easy to identify by a skilled gemologist except in the situation where the coating substance is colorless, and it has been added to improve durability.
4. Encountered in the trade – Occasionally for some gems. 5. Special care requirements – When they are not being worn, coated gem materials should be wrapped in soft packaging and kept in a dry environment.
Source: Robert Weldon, www.gia.edu
To learn more about gem treatments see also: