By Dr. James Shigley

Treatment processes are used to enhance the colour, clarity, durability, and appearance of gemstones, ensuring their saleability in a competitive market. While modern treatment methods guarantee the production of more attractive gems, their advancement ensures the challenging nature of treatment detection. Treatments are sometimes undertaken to deceive and defraud consumers. Depending on the treatment process, treated gems may not be durable in routine situations—such as normal jewellery wear and cleaning—and may be even more vulnerable to more extreme procedures such as heating during jewellery repairs.

As details of treatment processes are typically considered proprietary information, treatments are not always disclosed. The following discussion will focus on the types of treated diamonds likely to be encountered in the jewellery trade, and a summary of how these diamonds can be detected.

Clarity Treatments

1) Lasering dark mineral inclusions

Energy from a laser light is used to burn a tiny open channel from the surface of a diamond to a dark mineral inclusion. Liquid chemicals are then introduced under pressure through this channel, dissolving the inclusion or bleaching its dark appearance—so that inclusion remnants, while still present, are less obvious. The narrow straight channel to the inclusion can be seen with magnification.

An alternate approach is to focus one or more lasers on an inclusion and heating it. This can then be accompanied by the creation of a surrounding stress fracture. If the stress fracture extends to the diamond surface, it provides a channel for liquid chemicals to reach the inclusion. Again, such damaged inclusions can be seen with magnification, and the presence of stress fractures may make them even more apparent. This type of diamond treatment, introduced in the 1970s and 1980s, is permanent.

2) Glass filling open surface-reaching fractures or cleavages

Natural gem diamonds often contain internal or surface-reaching fractures that can reflect light and lower clarity grade. Special transparent lead glass can be introduced as a liquid, and pushed under pressure into narrow fractures that reach the diamond’s surface. This process was developed in the 1980s, and is currently the most widespread clarity treatment.

When the colourless filler glass is present, the fractures are much less visible, although still present—improving the overall apparent clarity of the diamond. When viewed with magnification, the glass-filled fractures often exhibit what gemmologists call diagnostic “flash effects colours” under particular lighting conditions. These colour effects arise from the close match of the glasses’ refractive index to that of diamond.

This glass-filling process is not permanent. If one of these treated diamonds is heated—for example, during jewellery repair—the filler glass can sustain damage, degrading the clarity of the diamond.

Colour treatments

Except for colour coatings on gems, the body colour of most transparent gems results from the selective absorption of incident light. “White” light (e.g. sunlight, artificial light) contains all the colours of the visible spectrum. When this light illuminates a coloured gemstone, some of the spectrum colour components of the light are selectively absorbed by the material, while the remaining components are transmitted by the gem. When recombined, these are perceived as the gem’s colour. For example, blue diamonds absorb most of the green to red components of the incident light, transmitting the blue component.

This selective light absorption results from the presence of impurities, such as minor amounts of nitrogen or boron, in the crystal structure of diamonds. Scientists refer to these impurities (or other conditions such as missing carbon atoms) as “optical defects” when they produce selective absorption of light—or absorption or emission features in the visible spectrum—of the material.

1) Surface coating

This is the application of a liquid substance—e.g. dyes, stains, inks, fingernail polish, or a thin-film solid material—to either a portion (often at the girdle or culet) or the entire surface of a polished diamond. This technique is the most ancient diamond treatment. The coating process is intended to produce a stronger face-up colour appearance to a near-colourless gemstone or, alternatively, to lessen the visibility of an unattractive colour—such as applying a blue stain to hide the faint yellow colour of a diamond.

Colour coatings on diamond often display visual evidence like scratches, abrasion, or areas missing the coating, all of which can be seen with magnification. Colour coatings are also not a permanent treatment. They may often be removed with the application of a solvent, such as alcohol or acetone.

2) Irradiation

Exposure to a source of radiation can produce colouration just beneath the surface or, if more energetic radiation is used, colour throughout the volume of the diamond. This radiation is most often exposure to electrons from a linear accelerator, or gamma rays in a nuclear reactor. Exposure to natural radiation sources from certain minerals can also produce colouration in diamonds. On rare occasions, an irradiated diamond can emit low-level short- or long-term residual radioactivity that may exceed safe exposure limits. Artificial irradiation is sometimes followed by the controlled heating of diamonds to produce additional colours.

Colour-treated diamonds sometimes exhibit localized irradiation colour zoning near the culet or other areas of the cut stone that can be seen with magnification. In most instances, more uniformly distributed colouration results from the irradiation process. Radiation exposure cannot be detected by standard gemmological testing methods (unless there is evidence of facet-related colour zoning), but it can be recognized using advanced spectroscopy techniques. Such testing confirms that the diamond has been irradiated but, on occasion, it can be unclear if the radiation originated from a natural mineral source or from artificial treatment. In such instances, the origin of colour is labelled as “undetermined.”

Irradiated coloured diamonds first began to appear in the market during the 1950s and 1960s. The irradiated colour can be damaged if the diamond is exposed to the heat, such as during jewellery repair. However, under conditions of normal jewellery use, the colours of irradiated diamonds are stable.

3) High-pressure, high-temperature (HPHT) heating

Exposure to very high temperatures in specialized high-pressure equipment can remove the brown colour of certain diamonds—rendering them colourless—or it can produce other colour changes throughout the stone. Except for visual evidence of heat damage to occasional solid inclusions or along fractures, HPHT colour treatment cannot be detected by standard gemmological testing, but it can normally be recognized with the use of advanced spectroscopy equipment. The colours of HPHT treated diamonds are stable under normal conditions of jewellery use and cleaning.

Diamonds that have had their clarity or colour improved by a treatment process are widely available in the marketplace, though some may not be correctly identified and disclosed. Before making an important diamond purchase, consumers are advised to request a grading and/or identification report from a major gem-testing laboratory.

Dr. James Shigley is a distinguished research fellow at the Gemological Institute of America (GIA) in Carlsbad, Calif. Prior to joining GIA in 1982, Dr. Shigley studied geology as an undergraduate at the University of California, Berkeley, and later received his doctorate in geology from Stanford University. He has authored articles on diamonds and other gemstones and is a well-known speaker on gemmological topics to both professional and general audiences. Dr. Shigley helps direct GIA’s research activities on the identification of natural, synthetic, and treated diamonds, as well as coloured gemstones.