Gemstone Treatments: A Clear Guide for Buyers

Most coloured gemstones sold today have been treated in some way, and that is normal. Gemstone treatments such as heating, oiling, filling and dyeing improve colour or clarity. Some are permanent and widely accepted, others are fragile and lower the value a lot, so the key is knowing which one your stone has and getting it in writing.

This guide walks you through each common treatment in plain words: what it does, how long it lasts, and how labs report it. Use it as a checklist before you pay for any important stone.

Why gemstones are treated at all

Very little rough comes out of the ground with top colour and clean clarity. Treatments let cutters turn pale, dark, milky or cracked material into stones people want to wear.

That is not a problem in itself. The problem starts when a treatment is hidden, or when a fragile treatment is sold at the price of a stable one.

• Treated is not fake: a heated sapphire is still natural sapphire. Fake means glass, plastic or a different stone pretending to be something else.
• Treated is not synthetic: a synthetic (lab-grown) gem is made in a factory. A treated gem was mined, then improved.
• Disclosure is the rule: the seller should tell you about any treatment, especially one that needs special care or changes the value.

Common gemstone treatments at a glance

Here is a quick table you can save. Details for each one follow below.

Treatment	Common in	What it does	How stable
Heating	Sapphire, ruby, tanzanite, aquamarine, citrine, zircon	Improves or changes colour, clears silk	Permanent in normal wear
Oil or resin	Emerald	Hides surface-reaching fissures	Not permanent; can dry out or wash out
Fracture filling	Diamond, emerald	Makes cracks less visible	Sensitive to heat and chemicals
Lead-glass filling	Low-grade ruby, some sapphire	Fills heavy fractures with glass	Fragile; damaged by acids and heat
Diffusion	Sapphire	Adds colour from outside the stone	Stable, but surface type can be polished off
Irradiation	Blue topaz, some diamonds, some tourmaline	Creates or deepens colour	Stable in topaz; some colours fade in light
Dyeing	Agate, onyx, jade, coral, lapis, chalcedony	Adds colour	Can fade or rub off with solvents
Coating	Topaz, quartz, some tanzanite and diamonds	Thin film adds colour or effect	Scratches and wears off
Impregnation	Turquoise, jade, opal	Resin or wax fills pores	Fairly stable; avoid heat

Heating: the most common and most accepted

Heating is the oldest and most widely used treatment. The stone is heated in a controlled furnace, which can change its colour, clear up fine needles (called silk) or both.

• Sapphire and ruby: heating improves blue, removes purple or brown tints and makes stones brighter. Most ruby and sapphire on the market has been heated.
• Tanzanite: almost all tanzanite is heated to bring out its blue-violet colour. We explain it in how tanzanite goes from brown to blue.
• Others: aquamarine (green to blue), citrine (often heated amethyst) and blue or colourless zircon.

Stability: heat results are permanent under normal wear. Still, avoid sudden intense heat, such as a jeweller's torch close to the stone, as with any gem.

Heating with residue

Some rubies are heated with a flux that leaves a small amount of glassy residue in fine cracks. Labs often note this as "minor" or "moderate" residue. A small amount is far less serious than full glass filling, which is covered below.

Oil and resin in emerald

Emeralds nearly always have tiny fissures that reach the surface. Filling them with colourless oil (traditionally cedarwood oil) or an artificial resin makes those fissures much harder to see.

• How common: the large majority of emeralds are clarity enhanced. Emeralds with no filler at all are rare and priced higher.
• Stability: not permanent. Oil can slowly dry out or be pushed out by heat, steam, ultrasonic cleaners or solvents. A stone can often be re-oiled by a professional.
• Lab wording: many labs grade the amount as minor, moderate or significant, and may name the filler type (oil or resin). Exact wording varies by lab.

Care matters here. See our emerald care guide before you clean one, and browse emeralds knowing the clarity grade on the report.

Fracture filling

Fracture filling puts a glass or resin into cracks so light passes through more smoothly. It is used on diamonds and emeralds.

• Sign to look for: a "flash effect" — a coloured flash (often blue, orange or pink) inside the crack when you tilt the stone.
• Stability: the filler can be damaged by heat, strong chemicals and harsh cleaning.
• Value: a filled stone is worth noticeably less than an unfilled stone of similar look.

Glass filling in ruby

This is the treatment that causes the most trouble for buyers. Very low-grade, heavily cracked ruby is filled with lead-rich glass. The glass hides the cracks and can make up a large part of the stone.

• Look for: a blue or orange flash in cracks, and flattened gas bubbles in the glass under a loupe.
• Care risks: acids, lemon juice, some household cleaners and a jeweller's torch can damage the glass.
• Value: far below a heated ruby. Some labs call these stones "composite" rather than simply ruby.

Lead-glass filling is also used on some low-grade blue sapphire, sometimes with a coloured glass. The same warnings apply.

Diffusion

Diffusion means heating a stone with chemicals so colour moves into it from outside. It is mostly seen in sapphire.

Surface diffusion

Elements such as titanium are diffused into the outer layer of pale sapphire to make it blue. The colour sits in a thin skin, so re-cutting or heavy re-polishing can remove it. In liquid, the colour often looks concentrated along facet edges.

Lattice (beryllium) diffusion

Beryllium can be diffused deep into sapphire to create yellow, orange or pinkish-orange colours. It is harder to detect and usually needs advanced lab testing. These stones are worth much less than sapphires with natural or heat-only colour.

Irradiation

Irradiation exposes a stone to radiation to change its colour. It is the reason blue topaz is so affordable.

• Blue topaz: almost all of it starts colourless and is irradiated, often followed by heating. The colour is stable.
• Diamonds and tourmaline: irradiation can create or deepen colour in some stones.
• Fading: some irradiated colours fade in bright light. Irradiated diamonds should be kept away from a jeweller's torch.
• Safety checks: in many countries, irradiated stones must meet radiation limits before they are sold.

Dyeing

Dye adds colour to porous or cracked stones. It is common in agate and onyx, chalcedony, jade, coral, lapis lazuli and some quartz.

• Warning signs: colour gathered in cracks or pits, colour that looks too even, or colour on a cotton bud after a gentle wipe.
• Stability: dyes can fade in sunlight and may come off with alcohol, acetone or nail polish remover.
• Do not test at home with solvents on a stone you do not own yet. Ask the seller or a lab.

Coating

A coating is a very thin film on the surface. It can add colour, a rainbow effect (as in "mystic" topaz) or hide a weak colour.

Stability: coatings scratch easily, especially along facet edges, and can wear off with time. Keep coated stones away from hard knocks and harsh cleaners, and store them wrapped separately.

Impregnation of turquoise

Much turquoise is soft and chalky. Filling the pores with a clear resin or plastic, called stabilisation, makes it harder, less likely to absorb skin oils and deeper in colour.

• Very common: a large share of turquoise in jewellery is stabilised. Natural untreated turquoise of good quality is less common and priced higher.
• Know the difference: "stabilised" is real turquoise with resin. "Reconstituted" is turquoise powder bonded into a block. Dyed howlite or magnesite is an imitation, not turquoise at all.
• Care: avoid heat, perfume and chemicals.

Read how to spot real feroza and see turquoise with this in mind. Jade and opal can be impregnated with resin or wax too.

Which treatments are stable?

A simple way to think about it:

1. Stable in normal wear: heating, lattice diffusion, irradiation in blue topaz, stabilised turquoise (with basic care).
2. Needs care and may need upkeep: emerald oil or resin, fracture filling, dyeing.
3. Fragile: coatings, lead-glass filling, surface diffusion if the stone is ever re-cut.

The more fragile the treatment, the more the price should reflect it.

How labs report treatments

A lab report is your best protection. Learn how to read one in our certifications guide. You will usually see wording such as:

• "Indications of heating" or "Heated": standard heat treatment was detected.
• "No indications of heating": the lab found no signs of heating with its current methods. This is the wording buyers mean by "unheated".
• "Heated, with minor (or moderate) residue": flux residue in fissures.
• Clarity enhancement grade: for emeralds, often minor, moderate or significant.
• "Composite", "lead-glass filled" or "manufactured product": heavy glass filling.
• Comments section: read it. Important notes, such as diffusion or dye, often appear here.

Remember that a report reflects what can be detected on the testing date. Some treatments are hard to prove, so reports from recognised labs carry more weight.

Disclosure: what you should expect

GIA's introduction to gem treatments sums up the standard: anyone selling a gem should disclose the treatments it has received. Trade bodies and consumer rules in many countries say the same.

Before you buy, ask these four questions and get the answers in writing:

1. Is this stone treated? If yes, how?
2. Is the treatment permanent?
3. Does it need special care?
4. Is there a lab report, and from which lab?

Ask for the lab report before you pay. At Raza Gems we show it on major stones, and you can learn more on our gemstone authentication page.

FAQs

Are treated gemstones fake?

No. A treated gemstone is still a natural mined stone that has been improved. A fake is a different material, such as glass, pretending to be the gem.

Is heat treatment acceptable in gemstones?

Yes, it is widely accepted in the trade because it is permanent and stable. It should still be disclosed, and unheated stones of similar quality usually cost more.

Which gemstone treatments are permanent?

Heating, lattice diffusion and irradiation of blue topaz are considered stable in normal wear. Oiling, dyeing, coating and glass filling are not fully permanent and need careful handling.

Can I tell if a gemstone is treated at home?

You can sometimes spot clues, such as a flash of colour in cracks, gas bubbles or dye in pits, using a 10x loupe. Most treatments, though, need a gem lab to confirm.

Does treatment lower a gemstone's value?

Usually, yes, but by very different amounts. A heated sapphire keeps strong value, while a glass-filled or coated stone is worth far less than its looks suggest.