Understanding Gemstone Treatments
The vast majority of colored gemstones on the market today have been treated or enhanced in some way to improve their color, clarity, or durability. Many of these treatments have been practiced for centuries and are considered standard in the gem trade. Understanding what treatments are common, which are permanent, and how they affect value will help you make informed purchasing decisions.
USA Jewels discloses all known treatments on our gemstones. We believe transparency builds trust and helps you buy with confidence.
Common Gemstone Treatments
Heat Treatment
What it does: Permanently improves color and/or clarity by heating the gemstone to high temperatures (typically 800–1800°C).
Which gems: Sapphire, ruby, tanzanite, aquamarine, citrine (from amethyst), blue zircon, tourmaline
Industry acceptance: Universally accepted. Heat treatment is so standard that an unheated sapphire or ruby of fine color commands a significant premium specifically because it is unusual. Virtually all tanzanite and blue zircon on the market is heat-treated. Most citrine is actually heat-treated amethyst.
Permanence: Permanent. The color will not fade or revert.
Effect on value: Heat-treated gems are priced as the standard. Unheated gems of equivalent quality command a 20–100%+ premium.
Oiling & Resin Filling
What it does: Fills surface-reaching fractures with oil (traditionally cedar oil) or synthetic resin to improve apparent clarity and reduce the visibility of inclusions.
Which gems: Emerald (nearly universal), sometimes ruby
Industry acceptance: Widely accepted for emerald. It has been practiced for centuries and is considered part of standard emerald care. The degree of treatment (none, minor, moderate, significant) is reported by major gem labs and affects value.
Permanence: Semi-permanent. Oil can dry out over time, especially if exposed to heat, ultrasonic cleaners, or chemicals. Re-oiling is possible. Resin fillers are more durable than oil.
Effect on value: Untreated or minimally oiled emeralds command the highest premiums. Heavily oiled emeralds are significantly less valuable.
Irradiation
What it does: Exposes the gemstone to radiation (gamma rays, electron beams, or neutrons) to create or change color, often followed by heat treatment to stabilize the result.
Which gems: Blue topaz (virtually all), some colored diamonds, some pearls, some tourmaline
Industry acceptance: Fully accepted for blue topaz. Sky Blue, Swiss Blue, and London Blue topaz are all produced by irradiating colorless topaz, and this is the standard. For diamonds, irradiated color-treated stones must be disclosed and are priced lower than naturally colored diamonds.
Permanence: Permanent when followed by heat stabilization.
Effect on value: For topaz, irradiation is standard pricing. For diamonds and other gems, it significantly reduces value compared to natural color.
Surface Coating
What it does: Applies a thin film of material to the surface to change the apparent color.
Which gems: Mystic topaz, some tanzanite, some quartz
Industry acceptance: Accepted only when fully disclosed. Coatings can wear off over time, especially with cleaning or normal wear.
Permanence: Not permanent. Coatings can scratch, peel, or wear away.
Effect on value: Significantly reduces value. Coated gems should be priced much lower than naturally colored or heat-treated equivalents.
Diffusion Treatment
What it does: Introduces color-causing elements (such as beryllium or titanium) into the surface or bulk of the gemstone through high-heat chemical diffusion.
Which gems: Sapphire (beryllium diffusion for padparadscha-like colors, titanium diffusion for blue)
Industry acceptance: Must be disclosed. Surface diffusion (color only on the surface) is less accepted than bulk diffusion (color throughout the stone). Both significantly affect value.
Permanence: Permanent, but surface diffusion color can be removed if the stone is re-cut or chipped.
Effect on value: Significantly reduces value compared to naturally colored or simply heat-treated stones.
Stabilization
What it does: Impregnates porous gemstones with wax, resin, or polymer to improve durability, color, and appearance.
Which gems: Turquoise (very common), lapis lazuli, some opals
Industry acceptance: Widely accepted for turquoise. Most commercial turquoise is stabilized because natural turquoise is often too soft and porous for jewelry without treatment. Untreated, high-quality turquoise commands a significant premium.
Permanence: Permanent under normal wear conditions.
Effect on value: Stabilized turquoise is standard pricing. Natural, untreated turquoise is much more valuable.
Dyeing
What it does: Introduces artificial color through dyes or chemicals.
Which gems: Black onyx (standard), some pearls, some jade, some agate, some lapis lazuli
Industry acceptance: Accepted for black onyx (it is the standard). For other gems, dyeing must be disclosed and generally reduces value. Dyed jade and pearls are considered significantly less valuable than naturally colored equivalents.
Permanence: Generally permanent for onyx. Can fade or bleed in some other gems.
Lead Glass Filling
What it does: Fills fractures in rubies with lead glass to dramatically improve apparent clarity and weight.
Which gems: Ruby (low-grade material)
Industry acceptance: Controversial. Must always be disclosed. Lead glass-filled rubies are not considered equivalent to natural rubies. The glass can deteriorate with exposure to acids, heat, or even common household chemicals like lemon juice.
Permanence: Not stable. The glass fill can break down, cloud, or fall out over time.
Effect on value: Dramatically reduces value. Lead glass-filled rubies sell for a tiny fraction of the price of natural rubies.
Treatment Summary by Gemstone
What to Ask Before Buying
- Has this gemstone been treated? A reputable jeweler will always disclose known treatments.
- What type of treatment? Heat treatment is standard; glass filling is not.
- Is the treatment permanent? Heat treatment and irradiation are permanent. Oiling and coatings may require maintenance.
- How does the treatment affect value? Know whether you are paying the right price for the treatment level.
- Does the stone come with a lab report? Major gem labs (GIA, AGL, Gubelin) identify and report all detectable treatments.
For more information on individual gemstones and their treatments, visit our Gemstone Buyers Guide.