Diffusion-treated
Diffusion treatment introduces colouring elements such as Ti, Be or Cr into a gem by high-temperature diffusion. Surface diffusion reaches only the surface layer, and bulk or lattice diffusion reaches the lattice. Diffusion-treated material must be disclosed.
On this page
Naming rule
- Naming rule
- Diffusion-treated [material name]
- Market differences and mutual recognition
- Some traders still sort diffusion into surface and lattice types, but that split is not in the texts on file: AGTA's 17th edition has a single code U, and LMHC Information Sheet #2 folds what used to be called surface diffusion into (lattice) diffusion, adding “shallow” when the colour is shallow. GB/T 16552 treats diffusion as a treatment, so the method goes with the name.
Gem treatments compares how every treatment on this site is named and what each must disclose.
Where the treatment acts (schematic)
Introduces colouring elements (Ti, Be, Cr etc.) into the surface layer (surface diffusion) or the lattice (bulk/lattice diffusion) by high-temperature diffusion.
- Colourless or light-coloured sapphire
- Alumina powder with minor titanium and iron oxides
- Alumina crucible
- Extended high-temperature heating
- Diffused blue surface layer
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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 1Diffusion treatment (sapphire as the example)
Before: Classic blue diffusion treatment starts from a faceted colourless or light-coloured sapphire.
During: The stone is embedded in alumina powder with minor titanium and iron oxides, in an alumina crucible, and heated in a furnace for an extended time.
After: The blue is confined to a shallow surface layer around a near-colourless core.
Treated zone: surface layer
Scene 2Process conditions
The stone is embedded in alumina powder with minor titanium and iron oxides, in an alumina crucible, and heated in a furnace for an extended time. Aluminium oxide is the major part of the powder, titanium oxide a minor part and iron oxide less; the cited patent gives 2 to 200 hours, usually at about 1600–1850°C (1700–1800°C preferred). After the heating the surface is rough, corroded and pockmarked, so the stone needs a light repolish, which may remove the diffused colour from the girdle or some facets.
Scene 3How it works
Iron and titanium colouring agents are first brought into contact with the surface; heating, sometimes to just below the melting point, expands the corundum lattice and lets the thermally energized transition-metal ions migrate into a surface layer, concentrating colour near the surface (the 1990 paper's simplified account). The blue itself comes from iron–titanium charge transfer: per a 2020 study, iron and titanium occupy neighbouring aluminium sites as Fe²⁺–Ti⁴⁺ pairs, and neither Fe²⁺ nor Ti⁴⁺ alone absorbs in the visible range.
Scene 4After treatment and how it is detected
The blue is confined to a shallow surface layer around a near-colourless core. In the three sectioned stones studied, measured after repolishing at the table edge, the colour layer was 0.15–0.42 mm thick; the thickness varies with the heating conditions, the polishing and where it is measured.
Immersed in methylene iodide and viewed in diffused transmitted light, every diffusion-treated stone studied showed colour concentrated along facet junctions and around the girdle, or localized, blotchy colour (or both). Prominent facet junctions alone do not settle it: the study also notes that sapphires with abraded facet junctions, and some natural untreated sapphires, may show a similar outlining in immersion.
How the naming authorities record it
This table scrolls sideways on narrow screens.
| Column | Value |
|---|---|
| Description | Introduces colouring elements (Ti, Be, Cr etc.) into the surface layer (surface diffusion) or the lattice (bulk/lattice diffusion) by high-temperature diffusion. |
| GIA code | U; Surface diffusion; Lattice/Bulk diffusion |
| LMHC number / note | U (CIBJO Gemstone Book 2022 Annex B, clause 7.2: Diffusion treatment); LMHC IS2 (corundum lattice diffusion only) - critical distinction: surface diffusion (colour at or close to the surface; reported as “shallow”) vs lattice/bulk diffusion (penetrates throughout) |
| GB/T category | Treatment (GB/T 16552-2017 §5.4.1 Table 1: diffusion treatment is classified as treatment, remark cell empty; representation method at §5.4.2.2). |
| CIBJO disclosure requirement | CIBJO Gemstone Book 2022 §5.79 defines Surface diffusion. CIBJO Gemstone Book 2022 §4.2.5.2 sets out the disclosure method (trade code U per Annex B, clause 7.2). |
| 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) diffusion meets. AGTA's manual (17th ed., p.V) states that failing to disclose diffusion violates the FTC Guides — that is AGTA's reading, not Part 23 text. |
| 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 diffusion (chemical treatment) (item ⑤) 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 | U — chemicals with high temperature to create artificial colour or asterism (GIM 17th ed., p.V). Where the induced colour does not permeate the whole stone, an added statement that recutting or repolishing is not recommended is required (p.V). The 17th edition has no separate surface / bulk diffusion codes. Consumer disclosure must be in plain language; codes alone are not sufficient (p.III). |
| Gem-A position | The Gem-A By-Laws have no diffusion-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 | Diffusion-treated [material name] |
| Disclosure requirement | Must be disclosed. Surface diffusion produces colour only in a thin layer that repolishing can remove; bulk diffusion is permanent but still requires disclosure. |
| Typical materials treated | Sapphire (surface titanium diffusion, bulk beryllium diffusion), ruby, feldspar (diffusion-treated labradorite) |
| Market differences and mutual recognition | Some traders still sort diffusion into surface and lattice types, but that split is not in the texts on file: AGTA's 17th edition has a single code U, and LMHC Information Sheet #2 folds what used to be called surface diffusion into (lattice) diffusion, adding “shallow” when the colour is shallow. GB/T 16552 treats diffusion as a treatment, so the method goes with the name. |
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:
The laboratory testing page's LMHC index lists every LMHC information sheet on report wording.
CIBJO definitions
- Diffusion
the diffusion of colour-causing or phenomenon-causing elements into a stone. See clause 4.2.5.2.
CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §5.24, p.22
- Surface diffusion
applying a combination of high temperature and chemicals to cause the diffusion of colour-causing or phenomenon-causing elements into a stone at or close to its surface. See clause 4.2.5.2.
CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §5.79, p.28
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 | Diffusion | Improves colour or produces asterism | Treatment |
| Sapphire | Diffusion | Improves colour or produces asterism | Treatment |
| Spinel | Diffusion | Improves or changes colour | Treatment |
| Topaz | Diffusion | Changes colour and other aspects of appearance | Treatment |
| Feldspar | Diffusion | Improves or changes colour | Treatment |
JJA/AGL table entries relating to this treatment
This table scrolls sideways on narrow screens.
| Gem name | Colour | As written in the table | Entry on this site | Page |
|---|---|---|---|---|
| Ruby | brownish red to purplish red | diffusion heat treatment | Ruby | p.12 |
| [colour] Sapphire | any colour | diffusion heat treatment | Sapphire | p.12 |
| Sapphire | orangy pink to pinkish orange | diffusion heat treatment | Padparadscha Sapphire, Beryllium-Diffused Sapphire | p.12 |
| Star Ruby / [colour] Star Sapphire | any colour | diffusion heat treatment | Star Ruby, Star Sapphire | p.12 |
| Andesine | green | copper diffusion | Andesine | p.15 |
| Red Andesine | red | copper diffusion | Andesine | p.15 |
Other treatments
Sources
This table scrolls sideways on narrow screens.
| 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 | §5.4.2.2 | Source |
| CIBJO | CIBJO Gemstone Book | 2022 | §5.79 | 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 #2 | version 9, May 2025 | External-element lattice diffusion | 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 (U) | 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
Repolishing is the difference that matters: when the diffused colour is shallow (at or close to the surface) it can be polished away, while beryllium diffusion runs through the whole sapphire and is permanent. Penetration depth is a laboratory finding, not a counter observation.
What to ask for on the report
A corundum report following LMHC Information Sheet 2 says (lattice) diffusion and may name the element; it adds “shallow” when the created colour sits near the surface, and that word is the signal that repolishing could take the colour away.
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.
- Repolishing is the difference that matters: when the diffused colour is shallow (at or close to the surface) it can be polished away, while beryllium diffusion runs through the whole sapphire and is permanent. Penetration depth is a laboratory finding, not a counter observation.
- A corundum report following LMHC Information Sheet 2 says (lattice) diffusion and may name the element; it adds “shallow” when the created colour sits near the surface, and that word is the signal that repolishing could take the colour away.
- Must be disclosed. Surface diffusion produces colour only in a thin layer that repolishing can remove; bulk diffusion is permanent but still requires disclosure.
Questions about this treatment?
Disclosure practice for this treatment differs between authorities and markets, and a report does not always say what was done.