Heated / Heat-treated
Heat treatment alters a gem's colour, clarity or appearance by heating. Heating is the most common gem treatment, and its acceptance varies by material.
On this page
Naming rule
- Naming rule
- Heated [material name]
- Market differences and mutual recognition
- Heating is where the codes diverge most. GB/T 16552 counts it as an optimization, so a heated sapphire may simply be called sapphire; CIBJO Gemstone Book 2022 §4.2.5.1 still requires the buyer to be told, using the word “Heated”. FTC §23.24 asks a different question: is the effect permanent, does it need special care, does it significantly affect the price?
Gem treatments compares how every treatment on this site is named and what each must disclose.
Where the treatment acts (schematic)
Alters colour, clarity or appearance by heating; the most common gem treatment.
- Milky geuda corundum with "silk" inclusions
- Rutile needles (silk)
- Heating apparatus
- High-temperature heating
- Blue after heating
If you reuse this animation, please credit LumaLark Wiki and link to this pageLicense: CC BY 4.0
Scene-by-scene transcript (4 scenes, with sources)
The caption of each scene in the animation above (the first scene lists its three panel captions), with sources.
Scene 1Heat treatment (Sri Lankan geuda corundum as the example)
Before: The 1981 paper reports that geuda, a milky white to pale blue, cloudy corundum rough, is turned blue by combining silk removal with reducing heat.
During: The stone is held at high temperature for an extended time, the "silk" inclusions dissolve and the reducing atmosphere turns the colour blue.
After: In suitable material, reduction held long enough at a high enough temperature can penetrate throughout the stone and give a uniform colour.
Treated zone: up to the whole stone if conditions suit
Scene 2Process conditions
Here silk removal and reductive colour development happen together. The stone is held at high temperature for an extended time, the "silk" inclusions dissolve and the reducing atmosphere turns the colour blue. Silk removal is typically done at 1500–1700°C, followed by fairly rapid cooling; the reducing atmosphere comes from hydrogen, or from packing the stone in charcoal or another carbon-producing material so that combustion with only a little air produces carbon monoxide; blue development is perhaps also at 1500–1700°C. Heating in an oxidizing atmosphere instead gradually converts the iron to Fe³⁺ and lightens the blue.
Scene 3How it works
At high temperature the rutile needles dissolve back into the corundum lattice, so the light scattering by the "silk" inclusions weakens and no longer dilutes the colour; reducing heat converts only some Fe³⁺ to Fe²⁺. Per a 2020 study, the main blue chromophore in sapphire is the Fe²⁺–Ti⁴⁺ pair on neighbouring aluminium sites, and neither Fe²⁺ nor Ti⁴⁺ alone absorbs in the visible range.
Scene 4After treatment and how it is detected
In suitable material, reduction held long enough at a high enough temperature can penetrate throughout the stone and give a uniform colour. Suitable means containing enough iron and titanium; inherent banding from an uneven distribution of impurities is the exception.
Possible signs listed in the 1981 paper are internal stress fractures around inclusions, chalky fluorescence (green in the original), no iron absorption line at 450 nm, pockmarked facets or abnormal girdles, and blotchy colour banding; the paper also notes that none of these may be present in a given treated stone, or some may be present in stones that have not been treated. Later work adds that the narrow 450 nm peak comes from Fe³⁺–Fe³⁺ pairs and is seen only at higher iron concentrations, and in 2019 heating experiments some stones heated to 1000°C or more showed chalky blue fluorescence under short-wave ultraviolet.
Sources
- Heat Treating Ruby and Sapphire: Technical Aspects, by Kurt Nassau (Gems & Gemology, Fall 1981, Vol. 17, No. 3; full-text PDF)
- A Quantitative Description of the Causes of Color in Corundum (Gems & Gemology, Spring 2020, Vol. 56, No. 1)
- Madagascar Sapphire: Low-Temperature Heat Treatment Experiments, by E. Billie Hughes and Rosey Perkins (Gems & Gemology, Summer 2019, Vol. 55, No. 2)
How the naming authorities record it
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| Column | Value |
|---|---|
| Description | Alters colour, clarity or appearance by heating; the most common gem treatment. |
| GIA code | H |
| LMHC number / note | H / FAH (CIBJO Gemstone Book 2022 Annex B, clause 7.2: Heating / Flux assisted healing); LMHC IS1 / IS7 (corundum only - report wording for heating residues and indications of heating; does not apply to other materials) |
| GB/T category | Optimization (GB/T 16552-2017 §5.4.1 Table 1: heating is classified as optimization, remark cell empty; representation method at §5.4.2.1). |
| CIBJO disclosure requirement | CIBJO Gemstone Book 2022 §5.41 defines Heating; §5.31 defines Flux assisted healing of corundum. CIBJO Gemstone Book 2022 §4.2.5.2 sets out the disclosure method (trade codes H and FAH per Annex B, clause 7.2; FAH is described as “Heated in flux”). |
| FTC regulatory citation | §23.24 general rule: disclosure is required if the treatment is not permanent (a), creates special care requirements (b), or has a significant effect on value (c). §23.24 names no individual treatment, so Part 23 does not say which of (a)–(c) heating meets. |
| RAL / German declaration | No public source text: RAL 560 A5 is a paid document not on file on this site, so no RAL/DGemG wording is shown (unchecked) |
| JJA/AGL (Japan) treatment disclosure | JJA/AGL 2025 §1-2-1 lists heating (item ①) among the treatments applied to natural stones; §2-1-1: the treatment is to be stated to the extent it can be known. |
| AGTA code | H — heating to alter colour, clarity and/or phenomena (GIM 17th ed., Aug 2026, p.V); for corundum, HR (heating with residue) where residues are visible at 10x (p.V). Consumer disclosure must be in plain language; codes alone are not sufficient (p.III). |
| Gem-A position | The Gem-A By-Laws have no heating-specific rule. §7.e.iii: before purchase the buyer should be told if the stone is known to have been treated and, where relevant, how that may affect its durability; §7.b: CIBJO guidelines and definitions are taken as best practice. |
| Naming rule | Heated [material name] |
| Disclosure requirement | Heating is a common treatment, and acceptance varies by material. Whether heated ruby or sapphire must be declared separately depends on which code is followed (see the comparison on this page); unstable colour or foreign residues must be disclosed. |
| Typical materials treated | Corundum group (ruby, sapphire), quartz group (amethyst, citrine, smoky quartz), beryl group (aquamarine, morganite), zircon, tanzanite, topaz, tourmaline |
| Market differences and mutual recognition | Heating is where the codes diverge most. GB/T 16552 counts it as an optimization, so a heated sapphire may simply be called sapphire; CIBJO Gemstone Book 2022 §4.2.5.1 still requires the buyer to be told, using the word “Heated”. FTC §23.24 asks a different question: is the effect permanent, does it need special care, does it significantly affect the price? |
Materials on which it is recorded
How lab reports word it, by material
The standard report wording (LMHC) for this treatment is set out on these entries and sections:
- Corundum
- Padparadscha Sapphire
- Paraíba Tourmaline
- Tanzanite
- Amber
- When colour authenticity cannot be determined
The laboratory testing page's LMHC index lists every LMHC information sheet on report wording.
CIBJO definitions
- Flux assisted healing
a flux assisted heat treatment of corundum enabling the healing of fissures via a process of dissolution at high temperatures, and deposition upon cooling. As fissures are sealed, silica glass becomes trapped within the previous fissures and this glass is often referred to as residues. See “flux assisted healing” within the chart of clause 4.2.5.2.
CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §5.31, p.23
- Heating
modifying a stone by a thermal process, e.g. in a furnace, kiln or other heating apparatus to change its colour and or clarity. See clause 4.2.5.2.
CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §5.41, p.24
to heat a pearl or cultured pearl to a temperature that may alters its appearance.
CIBJO Blue Book — Pearl Book (Pearl Commission 2024-1) (CIBJO): §5.96, p.31
to heat a coral to a temperature that may alter its appearance.
CIBJO Blue Book — Coral Book (Coral Commission 2024) (CIBJO): §5.50, p.16
The books word this differently; each wording is given with its source.
Definitions are quoted verbatim from the cited CIBJO Blue Book; […] marks an excerpt, and the full definition is at the cited clause.
Materials listed for this method in GB/T 16552-2017 Table B.1
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| Material (row) | Method | Effect | Category |
|---|---|---|---|
| Ruby | Heating | Improves appearance | Optimization |
| Sapphire | Heating | Improves appearance | Optimization |
| Aquamarine | Heating | Improves colour | Optimization |
| Beryl | Heating | Improves colour | Optimization |
| Tourmaline | Heating | Improves colour | Optimization |
| Zircon | Heating | Improves or changes colour | Optimization |
| Topaz | Heating | Improves or changes colour | Optimization |
| Garnet | Heating | Improves colour | Optimization |
| Rock Crystal | Heating | Improves or changes colour | Optimization |
| Zoisite (Tanzanite) | Heating | Improves colour | Optimization |
| Andalusite | Heating | Improves colour | Optimization |
| Feicui, Jadeite | Heating | Improves or changes colour | Optimization |
| Chalcedony (Agate/Jasper) | Heating | Improves or changes colour | Optimization |
| Fluorite | Heating | Improves colour | Optimization |
| Shoushan Stone | Heating | Improves or changes colour | Optimization |
| Amber | Heating | Improves colour and other aspects of appearance | Optimization |
JJA/AGL table entries relating to this treatment
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| Gem name | Colour | As written in the table | Entry on this site | Page |
|---|---|---|---|---|
| Black Opal | dark to black | heating | Black Opal | p.9 |
| Citrine | yellow to reddish yellow | heating | Citrine | p.10 |
| Green Quartz | green | heating | Green Quartz, Prasiolite / Green Quartz | p.10 |
| Cornelian | red to reddish brown | heating / colouring | Carnelian | p.11 |
| Sardonyx | reddish-brown and white bands | heating / colouring | Sardonyx | p.11 |
| Tiger's Eye | yellowish brown, brownish golden yellow | heating | Tiger’s Eye | p.11 |
| Amber | yellow, brown | heating | Amber | p.11 |
| Amber | red | colouring and heating | Amber, Red Amber | p.11 |
| Amber | yellow, green | heating under pressure | Amber, Green Amber | p.11 |
| Amber | yellow / green | heating under pressure | Amber, Copal, Green Amber | p.12 |
| Ruby | brownish red to purplish red | no treatment listed (noted: no indication of heating) | Ruby | p.12 |
| Ruby | brownish red to purplish red | heating | Ruby | p.12 |
| Ruby | brownish red to purplish red | heating, with residues in fissures | Ruby | p.12 |
| Ruby | brownish red to purplish red | heating, with cavities filled by a transparent substance | Ruby | p.12 |
| [colour] Sapphire | any colour | no treatment listed (noted: no indication of heating) | Sapphire | p.12 |
| [colour] Sapphire | any colour | heating | Sapphire | p.12 |
| [colour] Sapphire | any colour | heating, with residues in fissures | Sapphire | p.12 |
| [colour] Sapphire | any colour | heating, with cavities filled by a transparent substance | Sapphire | p.12 |
| Sapphire | orangy pink to pinkish orange | no indication of heating | Padparadscha Sapphire | p.12 |
| Sapphire | orangy pink to pinkish orange | heating | Padparadscha Sapphire | p.12 |
| Star Ruby / [colour] Star Sapphire | any colour | heating | Star Ruby, Star Sapphire | p.12 |
| [colour] Zircon | colourless, red, yellowish brown, blue, yellow, green | heating | Zircon | p.13 |
| Tanzanite | blue to violetish blue | heating | Tanzanite | p.14 |
| Black Diamond | black | carbonisation (heating) | Diamond | p.14 |
| Pink Topaz | orangy pink, pink | heating | Topaz | p.14 |
| Green Tourmaline | green | heating | Verdelite | p.15 |
| [colour] Tourmaline | blue to green (copper) | heating | Paraíba Tourmaline | p.15 |
| Blue Tourmaline | blue | heating | Indicolite | p.15 |
| Aquamarine | blue, greenish blue | heating | Aquamarine | p.16 |
Other treatments
Sources
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| Authority | Document | Version | Clause | Source |
|---|---|---|---|---|
| GB/T | GB/T 16552-2017 | 2017 | §5.4.1 | Source |
| GB/T | GB/T 16552-2017 | 2017 | Table 1 | Source |
| GB/T | GB/T 16552-2017 | 2017 | §5.4.2.1 | Source |
| CIBJO | CIBJO Gemstone Book | 2022 | §5.41 | Source |
| CIBJO | CIBJO Gemstone Book | 2022 | §5.31 | Source |
| CIBJO | CIBJO Gemstone Book | 2022 | §4.2.5.2 | Source |
| CIBJO | CIBJO Gemstone Book | 2022 | Annex B, clause 7.2 | Source |
| FTC | 16 C.F.R. Part 23 | §23.24 | Source | |
| LMHC | LMHC Information Sheet #1 | version 6, May 2025 | Healed fissures | Source |
| LMHC | LMHC Information Sheet #7 | version 6, February 2023 | Indications of heating | Source |
| JJA/AGL | JJA/AGL definitions and nomenclature for gems (Japanese-language document, no official English title) | issued January 2015, revised November 2025 | §1-2-1 | Source |
| JJA/AGL | JJA/AGL definitions and nomenclature for gems (Japanese-language document, no official English title) | issued January 2015, revised November 2025 | §2-1-1 | Source |
| AGTA | AGTA Gemstone Information Manual | 17th edition, August 2026 | p.V (H) | Source |
| AGTA | AGTA Gemstone Information Manual | 17th edition, August 2026 | p.V (HR) | Source |
| AGTA | AGTA Gemstone Information Manual | 17th edition, August 2026 | p.III | Source |
| Gem-A | Gem-A By-Laws §7 Code of Practice (Gem Code) | 2009 revision | §7.e.iii | Source |
| Gem-A | Gem-A By-Laws §7 Code of Practice (Gem Code) | 2009 revision | §7.b | Source |
What the eye can and cannot tell
Flux residues in a flux-healed ruby are not natural inclusions, and heat treatment with flux must be disclosed to the buyer. Beyond that, what the eye can flag is colour that may not hold or residue that does not belong, and either has to be disclosed when present.
What to ask for on the report
LMHC Information Sheet 1 and Information Sheet 7 set the corundum wording: indications of heating, heating residues. A ruby or sapphire report should be read against it. Those sheets do not apply to other materials, so for quartz, beryl, zircon, tanzanite or topaz the paperwork follows the naming pattern Heated [material name], GB/T 16552 Table 1 classifying heating as optimization.
Before you buy: what to check
Work through these in order before paying. None of them replaces a laboratory report; they decide whether you need one.
- Flux residues in a flux-healed ruby are not natural inclusions, and heat treatment with flux must be disclosed to the buyer. Beyond that, what the eye can flag is colour that may not hold or residue that does not belong, and either has to be disclosed when present.
LMHC Information Sheet 1 and Information Sheet 7 set the corundum wording: indications of heating, heating residues. A ruby or sapphire report should be read against it. Those sheets do not apply to other materials, so for quartz, beryl, zircon, tanzanite or topaz the paperwork follows the naming pattern Heated [material name], GB/T 16552 Table 1 classifying heating as optimization.
- Heating is a common treatment, and acceptance varies by material. Whether heated ruby or sapphire must be declared separately depends on which code is followed (see the comparison on this page); unstable colour or foreign residues must be disclosed.
Questions about this treatment?
Disclosure practice for this treatment differs between authorities and markets, and a report does not always say what was done.