An Introduction to Gem Treatments

Robert Weldon
Irradiated Topaz
Jump To:

As a consumer, you will regularly encounter in the marketplace gems that have been treated to change their appearance. One topic that often comes up is whether a particular gemstone is or isn’t treated. In a sense, humans alter all gem materials after they are found in the earth in order to prepare them for use in jewellery. Natural gem crystals are transformed from their rough crystallographic form into the shapes, outlines and degrees of polish of the gemstones that we appreciate and wear as jewellery. These steps are and have always been the routine procedures used for manufacturing gemstones. Beyond traditional cutting and polishing, however, gems can often be treated in ways that are intended to alter their colour or clarity. In addition to enhancing their appearance, the process may also improve (or in some cases diminish) the gem’s durability. Because these treatments are not always apparent to the unpractised eye, and are sometimes difficult to distinguish even by experts, it is necessary and legally required for anyone selling a gem (including consumer-to-consumer trade) to disclose any treatment procedure that it may have received.


Non-disclosure of this treatment could cause a person to believe that a particular gemstone was of higher quality naturally, and therefore more valuable than it actually is. One added challenge is that treatments can be permanent, long lasting or short-lived under normal jewellery use. Treated gems may require special care by their owner. In the United States, the Federal Trade Commission has established a set of consumer guidelines outlining the need for treatment disclosures and special care requirements, and countries around the world either adhere to similar guidelines or have regulations of their own. In addition, several professional organisations, such as the American Gem Trade Association (AGTA), or the International Colored Gemstone Association (ICA), or The World Jewellery Confederation (CIBJO), have formulated specific guidelines that their members are required to follow regarding the disclosure of treatments in gem materials. The following glossary includes terms that are often used in gem treatment nomenclature, and that you may encounter when shopping for gemstones. Finally, treatments for gemstones are constantly being changed and refined, and the detection of these new treated gems is an important part of on-going gemmological research.
 
The following guide will give a short description of the treatment process, some gems for which the process is used, how easy or difficult the treatment is to detect for a trained gemmologist, how often the treated gem might be encountered in the jewellery trade and how durable the material is under normal handling procedures. Any special care instructions for these treated gems are also provided.

Bleaching

– a chemical used to alter / reduce a component of, or the entire colour, of a porous gem. Some gemstones are bleached and then dyed, a form of “combination treatment”.

1. The most commonly encountered bleached gems include:

Jadeite jade
– Jadeite is often bleached with acid to remove an unwanted brown component from the material. Bleaching in jade is typically part of a two-step process: because acid bleaching causes the material to become slightly porous or susceptible to breakage along fractures, it is then subsequently treated with polymer impregnation to fill these open spaces in order to produce a better overall appearance.

Untreated and treated jadeite

These sections of jadeite show the material as it looks before and after bleaching.

Pearls – All types of pearls are routinely bleached with hydrogen peroxide to lighten and improve their uniformity of colour.

Bleached and dyed cultured pearls
Cultured pearls are routinely bleached to achieve uniformity of colour.

Other materials – Some coral, chalcedony and tiger’s eye quartz may be bleached to lighten their colour.

2. Detectability – Bleaching as a one-step process is virtually impossible to detect in most cases. The second step, impregnation with polymer compounds, is easier to detect by a qualified gemmological laboratory using magnification and more advanced analytical techniques.

3. Encountered in the trade – Frequently in pearls and jadeite.

4. Durability factors – Acid bleaching causes a breakdown in the structure of most materials, so as a stand-alone treatment, leaves materials vulnerable to breakage. Most bleaching is followed by impregnation to improve durability and strengthen perceived colour

5. Special care requirements – Bleached gems tend to be more brittle, and they may be much more porous and thereby more absorbent of human oils and other liquids. It is suggested that pearls be kept in a soft, dry environment in order to avoid surface damage.

Surface Coating

– altering a gem’s appearance by applying a colouring agent such as 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 colour.

1. The most commonly encountered coated gems include:

Diamonds – Thin-film coatings are sometimes used on diamonds to change their colour. 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 colour of the ink used. More modern coating methods use thin films of metal oxide.

Surface coated diamonds
The intense pink colour of these three diamonds is the result of a surface coating.

Tanzanite – Although this is rarely used, tanzanites have been coated to improve the intensity of their blue-violet colour.

Natural and treated tanzanite
Classic tanzanite exhibits a deep, purplish blue colour (left). Pale tanzanite and other pale stone species are sometimes coated with an ink-like substance to try to deepen and improve their colour (right).

Topaz – Some colourless topaz is coated with metal oxides to create the appearance of a variety of different colours. 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 colour was confined to the surface of the gemstone.

Natural and treated topaz
Some natural topazes are colourless (top two), but they can be coated with metal oxides to produce a variety of metallic colours (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 colour.

Treated coral
This golden coral is the result of a two-step process: one to bleach away the dark colour (the coral branch has been partially dipped in bleach to result in the golden colour). The coral is then coated with a resin to deepen the tone and protect the coral.

Pearls – Some pearls have reportedly been treated with a colourless hard coating in an effort to improve durability.

Quartz – Occasionally, quartz is coated with metal oxides to create colours rarely seen in natural quartz.

Vapor Deposition Quartz
Vapour deposition can coat many types of gems with metal oxides. This thin layer can alter the colour 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 into contact with coated gems.

3. Detectability - Once suspected, the treatment is easy to identify by a skilled gemmologist except in the situation where the coating substance is colourless, 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.

Dyeing

– introducing coloured dyes into porous or fractured gems to change their colour. Such fractures are sometimes purposely induced by heating the gem so that an otherwise non-porous material can more readily accept the dye.
 
1. The most commonly encountered dyed gems include:

Pearls – Dye often improves the appearance of lower–quality natural and cultured pearls by enhancing their colour.

Dyed Pearls
Many pearls seen in the market are dyed, as demonstrated by the bottle of dye-soaked pearls (left) and the single pearl (right).

Other gem materials – The process has been used since ancient times for materials such as coral, turquoise, lapis lazuli, howlite, nephrite jade, chalcedony, quartz, emerald and ruby.

Natural and dyed chalcedony
Natural chalcedony (colourless sphere to left), may be dyed with a variety of colours to achieve deeply coloured materials. A slice of chalcedony (right) may be dyed with a variety of different colours. This sample was sliced further into sections that were all dyed different colours.

Bleached and Dyed Coral 146769 636x358
The coral on the left was initially bleached and then dyed.

2. Durability factors – When dye is applied to porous materials, their durability may be long-lived, but this is ultimately dependent on the stability of the dye itself. In gems with larger fractures, the dye can sometimes leak out under a variety of conditions. Many dyes can be removed if the gem comes into contact with a solvent such as alcohol or acetone. Some dyes are unstable with exposure to the ultraviolet in sunlight and can fade over time.

3. Detectability – A qualified gemmologist can detect dyed gems in most cases.

4. Encountered in the trade – Occasionally for most gems, and frequently for coloured pearls.

5. Special care requirements – When it is known that gem materials have been dyed, care must be taken to not bring them in contact with chemicals such as acetone or alcohol, which could dissolve the dyes, or allow them to be exposed to prolonged periods of sunlight (such as leaving it on a sunny window ledge) which could cause the dyed colours to fade.

Fracture or cavity filling

– filling surface-reaching fractures or cavities with a glass, resin, wax or oil to conceal their visibility and to improve the apparent clarity of gem materials, appearance, stability, or, in extreme cases, to add a small amount of weight to a gem. The filling materials vary from solids (a glass) to liquids (oils), and in most cases, they are colourless (coloured filler materials could be classified as dyes).
 
1. The most commonly encountered fracture-filled gems include:

Diamond – Surface-reaching fractures are sometimes filled with high-lead-content glass. This reduces the visibility of the fracture, with the goal of enhancing the appearance of the diamond. The filled fracture is still present – it is just less apparent.

Fracture Filled Diamonds
Surface-reaching fractures in diamonds can be filled with molten lead glass, lessening the appearance of the fractures.

Ruby – Numerous surface-reaching fractures are filled with a glass to lessen their visibility and make the gem more transparent than it really is. In some cases, the amount of filler glass in a treated ruby can be significant.

Fracture filled rubies
Surface-reaching fractures in rubies such as these can be filled with molten lead glass, lessening the appearance of the fractures.

Emerald – Surface-reaching fractures in emerald are sometimes filled with essential oils, other oils, waxes, and “artificial resins” —epoxy prepolymers, other prepolymers (including UV-setting adhesives) and polymers to reduce the visibility of the fractures and improve the apparent clarity. These substances have varying degrees of stability in treated emeralds, and the volume of filler material present can range from insignificant to major amounts.

Fracture filled emeralds
Surface-reaching fractures in emeralds such as this one can be filled with artificial resins, wax and epoxy polymers. This lessens the appearance of the fractures, as the treated emerald on the right shows.

Other materials – Resins and glasses can potentially be used on any durable gem with surface-reaching fractures, including quartz, aquamarine, topaz, tourmaline and other transparent gems. This kind of treatment is, however, less prevalent than the other treatment processes mentioned above.

2. Durability factors – Much depends on the durability of the filler. Glasses tend to be harder and therefore more durable than resins, oils or waxes. Changes in air pressure, proximity to heat or exposure to chemicals can all affect the appearance of filled gems by potentially altering or removing the filler substance.

3. Detectability – In most cases, filled gems can be recognised by a qualified gemmologist using magnification.

4. Encountered in the trade – Often encountered for diamonds, ruby and sapphires and emerald.

5. Special care requirements – Avoid exposure to heat, and changes in air pressure (such as in an aeroplane cabin) or chemicals. Filled emeralds can also be damaged by exposure to hot water used for washing dishes.

Heat treatment

– the exposure of a gem to high temperatures for the purpose of altering its colour and/or clarity.

1. The most commonly encountered heat-treated gems include.

Amber
– When amber is submerged in hot oil—linseed oil for example—it's inherent body colour can darken, and the material can take on a clearer appearance. The hot oil can also cause the material to develop a series of spangled, glittery inclusions.

Heat treated amber
The rounded inclusions in amber are caused by immersing it in heated oil, which results in a “spangled” effect.

Amethyst – Heating can remove unwanted brownish inclusions in some amethysts or lighten the colour of overly dark stones.

Aquamarine – Without treatment, much of the aquamarine is blue–green in colouration. Heating in a controlled environment can remove the greenish colour component from the material to produce a more blue appearance.

Citrine – Some forms of amethyst can be heated and turned into citrine.

Ruby – Heating can remove purplish colouration rendering a more pure red colour. The process can also remove “silk” (minute needle–like inclusions) that can cause a gem to appear lighter in tone and be more opaque. Heating can also cause recrystallisation of the silk inclusions to make them more prominent, which allows the gemstone to have stronger asterism (a reflecting star effect).

Sapphire – Heating can intensify, or even induce, a blue colouration in sapphires. The heating can also remove “silk” inclusions, which also helps to make the material appear more transparent. It in can also cause recrystallisation of the silk inclusions to make them more prominent, which allows the gemstone to have stronger asterism (a reflecting star effect).

Heat treated sapphires
Pale sapphires that were once discarded in the mining process were treated to a desirable colour of blue when heated in a controlled environment.

Tanzanite – The mineral zoisite, which includes the gem variety known as tanzanite, it is often heated at low temperatures to remove a brownish colour component to produce a stronger purplish-blue colour.

Heat treated tanzanite
Tanzanite is often mined as a brownish material (as the rough and cut stones on the left show). Once heated, the gem changes to a blue or purplish blue colour (as the rough and cut gems on the right show).

Topaz – Heating yellowish pink topaz sometimes has the effect of removing the yellowish colour component, thereby intensifying the pink colour. Heating is also used to control the colour of blue topaz: material that may have started out colourless is irradiated followed by heating, which results in a desirable blue colour.

Heat treated topaz
This imperial topaz crystal was sawn in half. The crystal on the right was heated, resulting in the purplish pink colour. Both colours are highly desired in the market.

Tourmaline – Sometimes heat treating can cause excessively dark green material to become lighter in tone, or it may affect the colour in other tourmalines.

Zircon – Some reddish brown zircons are heated in controlled environments to produce more commercially viable colours, including an intense blue.

2. Durability factors – Heat treatments in all of the gemstones mentioned above are considered durable and permanent under normal handling conditions.

3. Special care requirements – Submitting gemstones to intense heat may render them slightly more brittle than usual, and care must be taken not to damage pointed faceted corners and edges.

High pressure, high temperature (HPHT) treatment

– Heating a diamond to high temperatures under high confining pressures to remove or change its colour.
 
Heating diamonds at high pressures and high temperatures can remove or lessen their brownish colouration so that the gem becomes colourless. Other types of diamond may be transformed from brown to yellow, orangey yellow and yellowish green, or to blue colours by this process.

HPHT Diamond
High pressure, high temperature treatments can alter the atomic structure of some types of diamond, in this case removing the brownish colouration and turning the diamond colourless.

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

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

3. Encountered in the trade – Occasionally in colourless diamonds, more readily in some coloured diamonds.

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

Impregnation

– 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.

Impregnated turquoise
Porous gem materials such as this pale turquoise on the left are impregnated with a wax or polymer substance, which in this case causes the material to deepen in colour and become more stable.

1. Durability factors – Many impregnations are often “skin deep” and, because of 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 gemmologist 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 jeweller's torch, since these are likely to damage the material.

Irradiation

– exposure of a gem to an artificial source of radiation in order to change its colour. This is sometimes followed by a heat treatment to further modify the colour. This second step is also known as a “combination treatment".

1. The most commonly encountered irradiated gems include:

Diamond – Neutron and electron radiation are the most common forms of artificial irradiation, and it is possible to induce black, green, blue green, deep yellow, orange, pink and red diamonds (often combined with a secondary step of heating, to achieve certain colours).

Irradiated Diamonds
Colourless and other diamonds (left) can be artificially irradiated causing a variety of colours. Some of the irradiated colours are then heated as a second step, resulting in additional colours (group right).

Corundum – Some bright orange colours are induced in sapphires with a pale yellow natural colour. The colour in these gems is not stable, and fades upon exposure to light.

Topaz – colourless topaz has little commercial value in the gem market today, but it can be subjected to artificial radiation to dramatically change its colour. Used in conjunction with heat treatment, a variety of strong blue colours can be achieved in topaz.

Pearl – Some pearls are irradiated resulting in dark grey colours.

Quartz – Varieties of quartz may be irradiated to produce amethyst, and some combination treatments that include heating after irradiation resulted in green quartz.

Other gems – Some varieties of beryl and spodumene can be irradiated to deepen an inherent colour, or change the colour altogether.

2. Durability factors – some irradiated gems’ colour fades upon exposure to strong light. Blue topaz, diamond and quartz tend to have very stable colours as long as they are not exposed to high temperatures (this is especially true for irradiated coloured diamonds, whose colours may be damaged if the diamond is exposed to the heat of a jeweller’s torch during jewellery repair procedures).

3. Detectability – Because strong blue colours do not occur naturally in topaz, such stones are considered to have undergone irradiation treatment. Strong colours in green, pink, and red diamonds should also be considered suspect. Determination whether a coloured diamond is natural colour or treated colour requires examination by an experienced gem-testing laboratory.

4. Encountered in the trade – Extremely frequent for topaz, and frequent in fancy colour diamonds.

5. Special care requirements – In the beryl and spodumene gemstones, the irradiated colour tends to be short lived and fades upon exposure to bright light. Otherwise, there are no special care requirements for most irradiated gem materials.

Laser drilling

– this involves using a narrow focused beam of laser light to burn an open channel from the surface of a diamond to reach dark inclusions. This is generally followed by the use of a chemical forced into the channel to dissolve or alter the appearance of the inclusion.
 
Diamonds are the only gemstones to be treated in this fashion, in part because only they can withstand the heat of a laser.

Laser Drilled Diamond
Three laser drill holes have been introduced through the table facet of this diamond in an attempt to improve the clarity. However, it appears as though the drill holes have created a significant cleavage crack around the inclusion which has actually made the inclusion more obvious. This type of treatment isn’t always successful at improving the clarity. Field of view is 4.4mm.

1. Durability factors – While lasers could potentially affect the structure of a diamond, most laser drill holes are microscopic, and the durability of the diamond does not become an issue.

2. Detectability – Easily detectable by most gemmologists and qualified gemmological laboratories because of the presence of the laser drill holes.

3. Encountered in the trade – Sometimes.

4. Special care requirements – There are no special care requirements for laser-drilled diamonds.

Lattice diffusion

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

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 colouring agents in corundum), the ability to fully penetrate the stone with colour met with little success. In 2003, very strongly coloured sapphires began to appear in the market, and diffusion was again suspected. It was found that it was indeed 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 colour. It was soon found that the colour of rubies could also be accentuated using this treatment process.

Lattice diffusion sapphires
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 colour. 

Lattice diffusion feldspar
Untreated rough feldspar (left) and various treated feldspars (right)

Other materials – There have been reports of diffusion to cause colour 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.

You Might Also Like

Resources For Retailers
Find a Retailer
learn more
Shop the Campus Store
Visit the campus shop
Learn More
Quality Assurance Benchmarks
Learn More
Gems & Gemology
G&G Spring 2016 Edition
Learn more