Gem treatments - Surface Coating

Surface Coating

– 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.

– Heating a diamond to high temperatures under high confining pressures to remove, or change its color.

Heating diamonds at high pressures and high temperatures can remove or lessen their brownish coloration so the gem becomes colorless. Other types of diamonds may be transformed from brown to yellow, orangy yellow and yellowish green, or to blue colors by this process.

High pressure, high temperature treatments can alter the atomic structure of some types of diamonds, in this case removing the brownish coloration and turning the diamond colorless.

1. Durability factors – HPHT treatments are considered stable and permanent to normal jewelry handling conditions.

2. Detectability – Difficult to identify, even by seasoned gemologists. If suspected, only a qualified gemological laboratory can confirm the treatment.

3. Encountered in the trade – Occasionally in colorless diamonds, more readily in some colored diamonds.

4. Special care requirements – Other than normal care considerations used for most jewelry, there are no particular instructions for the care and handling of HPHT treated diamonds.

– the surface of a porous gemstone is permeated with a polymer, wax or plastic to give it greater durability and improve its appearance.

The most commonly encountered wax or plastic impregnated gemstones are opaque, and they include turquoise, lapis lazuli, jadeite, nephrite, amazonite, rhodochrosite and serpentine.

Porous gem materials such as this pale turquoise on the left is impregnated with a wax or polymer substance, which caused the material to deepen in color and become more stable.

1. Durability factors – Many impregnations are often “skin deep” and due to the melting point of plastic and wax, can be susceptible to heat damage. Plastic impregnations are considered durable in gem materials such as turquoise as long as they are not subjected to heat or chemicals.

2. DetectabilityIn most instances a qualified gemologist can readily identify the treatment.

3. Encountered in the trade – Frequently seen in the trade.

4. Special care requirements – Care must be taken not to subject gemstones with wax or plastic impregnations to heat, such as that encountered by a jeweler’s torch, since these will likely damage the material.

– the penetration of certain elements into the atomic lattice of a gemstone during heat treatment, with the objective of changing or accentuating its color.

1. The most commonly encountered diffused gems include:

Corundum (ruby and sapphire) – while experimentation during the 1980s concentrated on diffusion of titanium and chromium (which are coloring agents in corundum), the ability to fully penetrate the stone with color met with little success. In 2003, very strongly colored sapphires began to appear in the market, and diffusion was again suspected. It was found that it was diffusion — but of a new element: beryllium. Beryllium which has a much smaller atom than titanium or chromium, was able to diffuse all the way through a sapphire; even large sapphires, successfully changing their color. It was soon found that the color of rubies could be accentuated as well using this treatment process.

Untreated sapphires on the left (first group), diffused and unpolished (second group), over polished needing re-diffusing (third group), and successful diffusion treatment (fourth group).

Feldspar – Varieties of feldspar, notably andesine and labradorite were found to be receptive to the diffusion of copper, completely altering their color.

Untreated rough feldspar (left) and various treated feldspar (right)

Other materials – There have been reports of diffusion to cause color alterations in both tourmaline and tsavorite (a variety of garnet) but the claims have not been substantiated.

2. Durability factors – The treatment is considered permanent.

3. Detectability – Extremely difficult to detect with certainty in many instances—and if so, only by qualified laboratories.

4. Encountered in the trade – Diffusion treated corundum is widespread in the trade.

5. Special care requirements – There are no special care requirements for diffusion treated corundum or feldspar

– 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:

- An introduction to Gem treatments

- Gem treatments - Bleaching

- Gem treatments - Lattice Diffusion

- Gem treatments - Laser Drilling

- Gem treatments - Irradiation

- Gem treatments- Impregnation

- Gem treatments - High Pressure, High Temperature (HPHT)

- Gem treatments - Heat treatment

- Gem treatments - Fracture or Cavity Filling

- Gem treatments - Dyeing