A gemstone treatment is any process, other than cutting and polishing, that changes a gem's color, clarity or durability. Treatments are extremely common. Most sapphires and rubies are heated, most emeralds are oiled or resin-filled, and most blue topaz is irradiated. A treated gem is still a natural gem, but treatment affects value, care and what a seller must tell you.
Why gems are treated
Mines produce far more low-grade rough than fine material. Treatments turn pale, cloudy or cracked stones into ones that look better and sell for more. Some treatments, such as heating corundum, have been practiced for centuries. Others, such as beryllium diffusion and lead-glass filling, appeared in the late 20th and early 21st centuries.
Some sellers use softer words for the same processes because they sound gentler. On this site, the plain word "treatment" is used throughout.
Common treatments
Heating
Heating is the most widespread treatment. Controlled heat can lighten or deepen color, remove unwanted tints, dissolve fine rutile silk to improve clarity, or change color completely.
Corundum is heated more than any other gem, often to very high temperatures. Heat turns brownish zoisite into blue-violet tanzanite, removes green from aquamarine, removes yellow from morganite, turns much amethyst into citrine, and turns brown zircon blue or colorless. Simple heat treatment is usually stable and permanent.
Heating with additives is a different matter. Fluxes can help heal fractures in ruby, leaving glassy residues. Heating with beryllium is classed as diffusion (see below).
Oiling and resin filling
Colorless oils, such as cedarwood oil, and synthetic resins are used to fill surface-reaching fractures, especially in emerald. The filler reduces how much light the cracks reflect, so they seem to fade. Oils can dry out or be removed by solvents and ultrasonic cleaning. Hardened resins are more stable but can still be damaged by heat or chemicals.
Glass filling
Low-grade ruby and sapphire are filled with high-lead glass. The glass fills fractures and cavities and greatly improves appearance. Heavily filled material is sometimes called composite ruby or composite sapphire, since much of the stone can be glass. The glass is easily damaged by heat and attacked by mild acids. Fracture filling of diamond with glass is covered on the diamond treatments page.
Dyeing
Dyes add color to porous or fractured materials. Black onyx is usually dyed chalcedony, and agate is commonly dyed blue, green, pink and other colors. Jade, turquoise, lapis lazuli, coral and pearls are also dyed. Fractured quartz is dyed to imitate other gems. Dyes can fade or rub off, and acetone on a cotton swab sometimes lifts them.
Irradiation
Irradiation creates color centers in the crystal. Most blue topaz is colorless topaz that has been irradiated and then heated. Irradiation can also produce yellow sapphire (often unstable), green or blue diamond, darker smoky quartz and colored pearls. Some irradiated colors are stable, while others fade in light or heat. Irradiated gems are checked to make sure residual radioactivity has fallen to safe levels before sale.
Diffusion
Diffusion uses high heat to push coloring elements into a gem from outside. Surface diffusion with titanium creates a thin blue layer on pale sapphire, which can be polished away. Beryllium lattice diffusion, which spread in the early 2000s, can penetrate a stone much more deeply and turns pale or pink sapphire into orange, yellow or padparadscha-like colors. The sapphire treatments page covers detection.
Coating
Coatings are thin films applied to a gem's surface to add color or iridescence. Mystic topaz has a thin metallic coating that gives a rainbow effect. Some diamonds are coated to mask a yellow tint or add color. Coatings can wear or scratch off and should always be disclosed.
Bleaching
Bleaching lightens or evens out color, usually with chemicals or hydrogen peroxide. Pearls and jadeite are often bleached. Bleached jadeite is usually then impregnated with polymer, because bleaching leaves it weaker.
Impregnation and stabilization
Porous gems are impregnated with wax, oil, plastic or resin to improve appearance and durability. Turquoise is commonly stabilized with resin. Bleached and polymer-impregnated jadeite is called B jade, and B jade that is also dyed is called C jade. Opal can be impregnated with sugar and acid or with polymers to darken it. The jade page explains the A, B and C system.
Stability
Treatments vary widely in how long they last and how much care they need.
| Treatment | Typical stability | Care notes |
|---|---|---|
| Heating (simple) | Stable | Normal care |
| Oiling | Not fully stable | Avoid ultrasonic, steam, solvents; can be re-oiled |
| Resin filling | Moderately stable | Avoid heat and harsh chemicals |
| Glass filling (ruby, sapphire) | Low stability | Avoid heat, acids, ultrasonic; tell your jeweler before repairs |
| Dyeing | Varies, can fade | Avoid solvents and harsh cleaners |
| Irradiation | Usually stable, some colors fade | Avoid long exposure to heat or light for some types |
| Lattice diffusion (beryllium) | Stable | Normal care |
| Surface diffusion (titanium) | Stable but shallow | Repolishing can remove color |
| Coating | Low stability | Avoid abrasion, ultrasonic and harsh cleaners |
| Impregnation and stabilization | Moderately stable | Avoid heat and solvents |
Treatments by gem
| Gem | Common treatments |
|---|---|
| Ruby | Heating, flux healing, lead-glass filling, dyeing, diffusion |
| Sapphire | Heating, beryllium diffusion, titanium diffusion, glass filling |
| Emerald | Oiling, resin filling |
| Diamond | Laser drilling, fracture filling, HPHT, irradiation, coating |
| Aquamarine | Heating |
| Tanzanite | Heating (almost always) |
| Topaz | Irradiation and heating (blue), coating (mystic), heating (pink) |
| Amethyst and citrine | Heating (much citrine is heated amethyst) |
| Tourmaline | Heating, irradiation (pink and red) |
| Zircon | Heating (blue and colorless) |
| Jadeite | Waxing (common, accepted for A jade), bleaching, polymer, dyeing |
| Turquoise | Stabilization, dyeing, reconstitution |
| Opal | Impregnation, smoke or sugar-acid treatment, doublets and triplets |
| Pearl | Bleaching, dyeing, irradiation |
| Agate and onyx | Dyeing |
| Lapis lazuli | Dyeing, waxing |
The ruby treatments page gives more detail for that gem.
Disclosure norms
In the United States, the Federal Trade Commission's guides for the jewelry industry say that sellers should disclose any treatment that is not permanent, that creates special care needs, or that has a large effect on value. Many other countries have similar rules, and trade groups such as CIBJO publish disclosure standards. Honest sellers disclose all known treatments.
Gem labs test for treatments and note them on reports. Common phrases include "indications of heating" and "no indications of heating." For emerald and other filled stones, reports describe the amount of clarity treatment, using terms such as minor, moderate or prominent. Unheated stones of fine quality sell at a premium, so buyers often ask for a lab report. The colored gemstone grading page explains what reports cover.
Frequently asked questions
Are treated gemstones fake?
No. A treated gem is still a natural stone, with its appearance changed by heat, filling or another process. A fake is a different material altogether, such as glass.
Do treatments lower a gem's value?
Usually, compared with an untreated stone of the same appearance. The effect is small for accepted, stable treatments such as heating corundum and larger for treatments like lead-glass filling.
How can I find out whether my gem is treated?
A gem lab can test for most treatments using microscopes and spectroscopy. Ask the seller for written disclosure and, for important stones, a lab report.
Is heat-treated sapphire stable?
Yes. Simple heat treatment of corundum is permanent and needs no special care. Glass-filled or surface-diffused stones need more caution.