Published: June 2012
Coating is a method of enhancement that can vary widely. It can be as simple as a spot from a permanent marking pen on a diamond’s girdle to nail polish on the underside of a stone. These are not recommended treatments, but they have been encountered in the jewelry industry. Most of us are familiar with foil-backs, which are usually glass with a foil backing to make the material more brilliant. These were common in older fashion jewelry. Centuries ago, this technique was used on natural gems to enhance color and brilliance, but this is not a common practice today, as we have better materials to work with. A small spot of color, applied with ink or under a prong, can enhance or diminish a stones apparent color and, therefore, its value. While most coatings are easily detected if you know what to look for, they can remain an effective deception because these types of treatment are not always checked.
Modern gem coatings are similar to the films used for eyeglasses and camera lenses. Coatings are applied in a very thin layer and do not adversely affect the optic properties of the gem. Some older methods of coating remain that are performed on beads and less expensive materials. Most of these older treatments are not recommended as they can easily wear off the material. Chemical vapor deposition (CVD) is the preferred method of applying coatings today. This process applies extremely small particles on a surface to form a thin, bonded layer or coating. There is commonly a bonding layer applied first, then the layer that adds the color or optical effect is added on top of the bonding layer. It is similar to car painting, but on a nanoparticle scale. The coatings can add color, such as the CVD pink topaz, or create a metallic film, such as mystic topaz. They can also create a variety of fancy colors in diamond. This technique is also used to mask a color that is less desirable. Applying a light blue coating to a diamond can improve the apparent color of a slightly yellow stone. Coatings of this nature can be applied to many materials and be of any color or combination of colors.
In most cases, the CVD coating is applied to the underside (pavilion) of a stone. This creates the desired effect, as well as protects the coating from damage when worn in jewelry. Most coatings will tolerate gentle care and handling, but some can be removed by ultrasonic cleaning. All coatings can be scratched off, but CVD process coatings are more durable. All coated gems require special care, in order to avoid damage to the coating. If closely observed, it is sometimes possible to detect a coating by its iridescent effect in reflected light, or in some cases, it is possible to see where the coating may have worn off. JTV carries only the more-durable, CVD coating process on its colored topaz and quartz.
Coatings allow for a wide range of colors and effects to be applied to natural stone materials. Colorless topaz can be coated blue to resemble blue topaz that is typically irradiated, and colorless quartz can be coated purple to resemble amethyst, but most often coatings create new colors and effects that cannot be found in nature. The future of this technology is looking at applying diamond coatings to gem materials in the hope of adding the hardness of diamond to the beauty and affordability of other gemstones. Although there is not yet a means detectable with gemological instruments that changes any measurable properties of the coated gem, the research continues.
Lacquering is the process of applying a clear outer layer to a material. A gem material may be dipped into, sprayed or painted with a clear coating. This treatment is commonly applied to shell and wood as well as some softer bead materials in order to protect them and prevent the absorption of oils or fluids that may cause discoloration.
Modified: April 2011
Published on JTV.com: January 2012
Published on JTV.com: January 2012