Humans have treated gemstones for thousands of years. Much of the gem material currently on the market has been treated in some way before it reaches the consumer. As a gemologist, trader, or consumer, you’ll want to know the most frequently used treatments so you know what to look for and what questions to ask before purchasing or working with gems.
Colorful specimens of agate are likely dyed. Concentrated color in cracks and fissures is a good indicator of dye.
Common treatments for amber include heat treatment, dye, and reconstitution. Coatings and fracture fillings are less common.
Heat treatments darken the color and leave “sun spots.” These spots appear like glitter within the stone.
For dye treatments, look for color concentration in pits and fractures and uneven color distribution.
Reconstituted amber consists of several pieces pressed or melted together. In some cases, the people creating reconstituted pieces add modern insects to the mix. Artificial resins as well as copal may also be added to the mix. Reconstituted gems often exhibit a mosaic structure, which may only be visible under high magnification.
Some amethyst undergoes heat treatment to lighten its color. However, this undetectable procedure is rare.
In other cases, purple dye in colorless quartz creates the observed color.
For pieces with both purple and yellow saturated colors, check for dye treatments.
Read more about aquamarine heat treatment.
Cat’s Eye (Chrysoberyl)
Cat’s eye stones rarely receive treatments. However, some stones may contain oil or dye, and others may undergo radiation treatment to improve their color.
Bleach and dye treatments are common in chalcedony. So, check any brightly colored samples carefully.
In addition, heat treatments can improve the color in certain stones.
Chalcedony can also receive coatings, sometimes only on the back of a cabochon.
There are no known treatments for chrome diopside.
Since natural-color citrine is rare, most of the material on the market has undergone treatment. In fact, heating amethyst produces most citrine, in strong yellow or orangey hues, while heating smoky quartz can produce lemon-yellow hues. These treatments are undetectable.
In some cases, colorless quartz with yellow dye may imitate citrine.
Treatments in diamond are rare. However, if you purchase a diamond, send it to a gemological laboratory to catch any possible treatments.
Diamonds may receive laser drilling, high-pressure/high-temperature (HPHT), coatings, fillings, and radiation treatments. Read more about these procedures in our articles on diamond treatments and HPHT treatments.
- “A” grade jade is untreated, but may have a coating of wax to improve its luster.
- “B” jade has undergone intense treatment. First, treaters soak the jade in acid to remove brown areas. Then, they impregnate the stone with polymer, resulting in improved transparency.
- “C” jade contains dye. Check for areas of concentrated dye.
In addition, some jade is treated with acid and dyed polymer. This is sometimes called “B+C” jade.
Read more about jade treatments.
Some jasper stones receive dyes to create more intense colors. Check for concentrated color in cracks and surface pits.
Dyes are commonly used to hide white calcite inclusions in lapis lazuli. In addition to dye treatment, most specimens contain a coating of wax or plastic to seal in the dye. In some cases, plastic or wax may even be present in undyed specimens.
Wax, resins, or polymers can help to hide surface cracks in malachite. Some epoxies also double as hardeners for this soft gemstone.
While moonstone is typically untreated, some specimens have artificial coatings. Dark coatings on the back of a cabochon can enhance adularescence. Some gems may also have a blue coating to enhance their color.
Heat treatment enhances the pink color in morganite. This common treatment is undetectable.
Dye treatments are common in onyx. Almost all black onyx stones receive a sugar and acid treatment to get their color.
Opal treatments are uncommon. However, matrix opal and hydrophane opal may receive treatments.
The matrix in matrix opals can undergo a sugar/acid treatment to get black body colors. Since they have crumbly textures, they may also undergo polymer treatment for stabilization.
Hydrophane opals can be dyed or smoked to alter their body colors. They may also have polymer or resin enhancements.
Any type of opal may be an assembled gemstone. Check for dark backings and clear caps.
Read more about these procedures in our in-depth guide to opal treatments.
Some pearls may also undergo non-routine treatments. These include color enhancements using dye, heat, or radiation, luster enhancements from buffing or coatings, and stabilization by epoxy filling.
Learn more about pearl treatments.
Gemstone treatments in peridot are highly uncommon. Still, there are occasional reports of epoxy fillings.
Any variety of quartz can undergo irradiation to form smoky quartz. Irradiation and heat can produce numerous colors, including yellow-green “lemon” quartz and yellow to red varieties.
Heat treatment of amethyst can produce prasiolite (green quartz) as well as citrine.
There are no known treatments of rose quartz.
Intensely colored quartz may contain dye. Keep in mind that dyed quartz can also imitate other, more expensive stones.
Read more about quartz color treatments.
Virtually all rubies on the market today receive heat treatments. While dissolved silk is evidence of this, some rubies lack these inclusions, making it impossible to determine whether they have been heated.
Many rubies also contain leaded glass in surface-reaching fractures to enhance their clarity. Gas bubbles may be visible under a microscope, and darkfield illumination can reveal blue and orange flashes in such rubies.
In addition, some sapphires receive a diffusion treatment that makes them red like rubies.
Like ruby, most sapphire undergoes heat treatment. Dissolved silk, when present, indicates heat treatment.
Some sapphires may also contain leaded glass in surface-reaching fractures. While this occurs less frequently in sapphire than ruby, you should still check for these treatments.
Cobalt or beryllium diffusion treatments can enhance sapphire color. Evidence of cobalt diffusion in blue sapphire can be seen with a Chelsea filter and by immersing the stone in RI fluid. Beryllium diffusion can create numerous colors, and the easiest way to detect this treatment is by immersion.
Irradiation can produce yellow and orange colors in sapphire.
Read more about sapphire treatments.
Sunstone and Oregon Sunstone
Although there are no known treatments for sunstones or Oregon sunstones, some feldspars may be enhanced with copper diffusion. This treatment gives them an intense red color similar to some natural-color Oregon sunstones.
Heat treatment produces the signature blue-to-violet color in nearly all tanzanites. This treatment is undetectable.
While some tanzanites may have filled fractures, this is uncommon.
Tiger’s eye may receive treatment to improve its color. Heat treatment can help develop red colors. Bleach can lighten darker stones. In addition, dyes can enhance or alter the gem’s color.
Heat and radiation treatments are common in topaz, and most topaz has undergone some treatment. These treatments can enhance traditionally prized yellow, orange, and red colors or create a range of blue colors more popular with contemporary jewelry.
Many red, pink, blue (including paraíba), and green tourmalines undergo heat treatment, some in combination with radiation treatment.
In addition, some paraíba tourmalines may undergo fracture-filling enhancements to improve clarity.
Numerous treatments exist for turquoise. Some turquoise stones (as well as turquoise imitations) contain dye. Some are reconstituted from small pieces and dust, then mixed with polymer to bind them. Others receive polymer or epoxy coatings to improve their hardness.
In addition, the proprietary “Zachery Treatment” involves soaking turquoise in a solution to reduce porosity and improve luster. This treatment is only detectable through spectroscopy.
Wax coatings are placed even on high-quality turquoise to prevent discoloration from skin oils.
Nearly all zircon undergoes heat treatment. Treatment can improve clarity and, in some cases, alter colors. Popular colorless and blue stones generally originate as brownish rough. For more information on zircon heat treatments, check out our zircon gemstone page.
Common Gemstone Treatments Cheat Sheet
If you’re new to gemology, keeping all the different treatments straight in your head can be difficult. The following cheat sheet is the perfect reference for common gemstone treatments. Click here for a printable version.