Coated
Coating applies a thin film, such as a metal oxide, polymer or diamond film, to the gem surface. Coating alters colour, lustre or appearance, or increases durability. Coated material must be disclosed.
On this page
Naming rule
- Naming rule
- Coated [material name] ([coating material])
- Market differences and mutual recognition
- GB/T 16552 is the only text here that lets some coatings through as an optimization: a colourless protective film on an organic gem, provided it is annotated. A coloured film, or any film on a mineral gem or jade, is a treatment. In the CIBJO Gemstone Book 2022, lacquering, enamelling, inking and foiling all fall under the one description “Coated”.
Gem treatments compares how every treatment on this site is named and what each must disclose.
Where the treatment acts (schematic)
Applies a thin film to the gem surface (metal oxide, polymer, diamond film etc.) to alter colour, lustre or appearance, or to increase durability.
- Colourless topaz (cross-section)
- Coating apparatus (schematic)
- Coating material (from the source to the stone)
- Pink film
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 1Coating (pink coated topaz as the example)
Before: The starting material here is a faceted colourless topaz.
During: A thin coloured film is deposited on the surface of the stone.
After: The colour sits only in a thin film on the surface, with colourless topaz beneath it.
Treated zone (shown here as pavilion-only): a surface film, colourless beneath
Sources
- Coloring of Topaz by Coating and Diffusion Processes: An X-ray Photoemission Study of What Happens beneath the Surface, by Harald Gabasch et al. (Gems & Gemology, Summer 2008, Vol. 44, No. 2, pp. 148–154; archived as issue PDF pages 44–50)
- Gem News: Surface-treated topaz (Gems & Gemology, Summer 1998, Vol. 34, No. 2, pp. 142–144; archived as issue PDF pages 66–68)
- Surface treatment of gemstones, especially topaz – an update of recent patent literature, by Karl Schmetzer (The Journal of Gemmology, 2008, Vol. 31, No. 1/2, pp. 7–13; archived as issue PDF pages 9–15)
- Coated Topaz, by Riccardo Befi, Lore Kiefert and Min Htut (Gems & Gemology, Fall 2006, Vol. 42, No. 3, p. 128, Gemological Research Conference abstract; archived as issue PDF page 130)
Scene 2Process conditions
A thin coloured film is deposited on the surface of the stone. Coating methods include chemical vapour deposition, physical vapour deposition and sputtering; the pink, orange and red material GIA examined in 1998 appeared to have been sputter-coated, and in the pink sample of a 2008 study the colour came from gold embedded in a silica (SiO₂) film. Some coatings, especially pink ones, were applied only to the pavilion.
Sources
- Gem News: Surface-treated topaz (Gems & Gemology, Summer 1998, Vol. 34, No. 2, pp. 142–144; archived as issue PDF pages 66–68)
- Surface treatment of gemstones, especially topaz – an update of recent patent literature, by Karl Schmetzer (The Journal of Gemmology, 2008, Vol. 31, No. 1/2, pp. 7–13; archived as issue PDF pages 9–15)
- Coloring of Topaz by Coating and Diffusion Processes: An X-ray Photoemission Study of What Happens beneath the Surface, by Harald Gabasch et al. (Gems & Gemology, Summer 2008, Vol. 44, No. 2, pp. 148–154; archived as issue PDF pages 44–50)
- Coated Topaz, by Riccardo Befi, Lore Kiefert and Min Htut (Gems & Gemology, Fall 2006, Vol. 42, No. 3, p. 128, Gemological Research Conference abstract; archived as issue PDF page 130)
Scene 3How it works
In this example the colouring components sit in the film: the colour is produced by the coating itself, not by changing the composition of the topaz. In the paper's general description of coating, the film meets the stone at a narrow interface and the two hardly intermix; the pink sample measured showed no appreciable interdiffusion. The abrupt change in refractive index between film and stone causes angle-dependent interference in visible light (400–700 nm), which may give the stone an artificial look, so the colour seen also depends on the film structure and viewing conditions. The pink sample in the 2008 study showed angle-dependent reflection to the unaided eye.
Scene 4After treatment and how it is detected
The colour sits only in a thin film on the surface, with colourless topaz beneath it. In the pink sample of the 2008 study, a gold-bearing silica layer lay about 200 nm deep, the composition changed to topaz at about 350 nm, and from about 400 nm down it was pure colourless topaz (depth calibration uncertain by about 20%); no appreciable diffusion was found between film and stone.
Durability differs from film to film and depends on the film's composition. The pink surface film GIA examined in 1998 scratched off easily with a pin, whole parcels of the red material at the Tucson show had obvious paper wear on the facet junctions, and under magnification the surface colour looked spotty. CIBJO warns that solvents, heat or abrasives can often strip a coating, and advises against recutting or repolishing a coated stone. Where a stone is measured matters too: in some titanium-bearing, pavilion-only coated topaz examined by AGTA in 2006, EDXRF from the table detected only topaz, and the coating showed up by its high titanium content only when measured from the side. X-ray photoelectron spectroscopy (XPS) depth profiling shows a sharp compositional step between film and stone, which separates coating from diffusion.
Sources
- Coloring of Topaz by Coating and Diffusion Processes: An X-ray Photoemission Study of What Happens beneath the Surface, by Harald Gabasch et al. (Gems & Gemology, Summer 2008, Vol. 44, No. 2, pp. 148–154; archived as issue PDF pages 44–50)
- Gem News: Surface-treated topaz (Gems & Gemology, Summer 1998, Vol. 34, No. 2, pp. 142–144; archived as issue PDF pages 66–68)
- CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022)
- Coated Topaz, by Riccardo Befi, Lore Kiefert and Min Htut (Gems & Gemology, Fall 2006, Vol. 42, No. 3, p. 128, Gemological Research Conference abstract; archived as issue PDF page 130)
How the naming authorities record it
This table scrolls sideways on narrow screens.
| Column | Value |
|---|---|
| Description | Applies a thin film to the gem surface (metal oxide, polymer, diamond film etc.) to alter colour, lustre or appearance, or to increase durability. |
| GIA code | C |
| LMHC number / note | C (CIBJO Gemstone Book 2022 Annex B, clause 7.2: Coating); LMHC has no matching Information Sheet |
| GB/T category | GB/T 16552-2017 §5.4.1 Table 1: coating has two tiers - a colourless film on the surface of a natural organic gem is optimization (annotation required); a colourless or coloured film on a natural gemstone or jade is treatment. Representation method: optimization at §5.4.2.1, treatment at §5.4.2.2 (see Table 1). |
| CIBJO disclosure requirement | CIBJO Gemstone Book 2022 §5.14 defines Coating. CIBJO Gemstone Book 2022 §4.2.5.2 sets out the disclosure method (trade code C 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) coating 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 coating (item ⑨) 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 | C — surface enhancements such as lacquering, enameling, inking, foiling or sputtered films (GIM 17th ed., 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 coating-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 | Coated [material name] ([coating material]) |
| Disclosure requirement | Must be disclosed. Coatings may be abraded, flake off or discolour, and coating is not a permanent enhancement; the coating material must be disclosed (e.g. Mystic Topaz is coated topaz), and the treated effect must not be implied to be natural. |
| Typical materials treated | Topaz (Mystic, Azotic coatings), diamond (CVD film), pearl (polymer coating), opal (resin coating) |
| Market differences and mutual recognition | GB/T 16552 is the only text here that lets some coatings through as an optimization: a colourless protective film on an organic gem, provided it is annotated. A coloured film, or any film on a mineral gem or jade, is a treatment. In the CIBJO Gemstone Book 2022, lacquering, enamelling, inking and foiling all fall under the one description “Coated”. |
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.
Care after this treatment
The CIBJO Gemstone Book §6.2 names this treatment in its special care: special-care code n.
CIBJO definitions
- Coating
a layer of a substance spread over the surface, or part of the surface, of a stone for protection, colouration, decoration or deception; a covering layer. See clause 4.2.5.2.
CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §5.14, p.21
an artificial layer of any natural or artificial substance spread over the surface, or part of the surface, of natural pearls and cultured pearls for protection, colouration, increased lustre and other optical phenomena (orient and overtone), decoration or to alter their appearance; a covering layer.
CIBJO Blue Book — Pearl Book (Pearl Commission 2024-1) (CIBJO): §5.44, p.26
an artificial layer of any natural or artificial substance spread over the surface, or part of the surface, of coral for protection, colouration, increased lustre, decoration or to alter their appearance; a covering layer.
CIBJO Blue Book — Coral Book (Coral Commission 2024) (CIBJO): §5.21, p.12
The books word this differently; each wording is given with its source.
- Foiling
the application of a very thin layer of highly reflective metal applied to all or part of the pavilion side or at the back of a gemstone with the intent of reflecting light back to the viewer’s eye. This reflective surface can be "mirror like" and possibly coloured.
CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §5.32, p.23
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
This table scrolls sideways on narrow screens.
| Material (row) | Method | Effect | Category |
|---|---|---|---|
| Diamond | Coating | Changes colour and other aspects of appearance | Treatment |
| Emerald | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Beryl | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Tourmaline | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Topaz | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Rock Crystal | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Feldspar | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Zoisite (Tanzanite) | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Feicui, Jadeite | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Opal | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Marble | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Fluorite | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Talc | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Chicken-blood Stone | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Shoushan Stone | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Qingtian Stone | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Balin Stone | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Changhua Stone | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Mica Jade | Coating | Improves or changes lustre, colour and other aspects of appearance | Treatment |
| Coral | Coating | Improves or changes lustre, colour and other aspects of appearance | Graded — see §5.4.1, Table 1 |
| Amber | Coating | Improves or changes lustre, colour and other aspects of appearance | Graded — see §5.4.1, Table 1 |
| Mammoth Ivory | Coating | Improves or changes lustre, colour and other aspects of appearance | Graded — see §5.4.1, Table 1 |
| Shell | Coating | Improves or changes lustre, colour and other aspects of appearance | Graded — see §5.4.1, Table 1 |
JJA/AGL table entries relating to this treatment
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 | §5.4.2.1 | Source |
| GB/T | GB/T 16552-2017 | 2017 | §5.4.2.2 | Source |
| CIBJO | CIBJO Gemstone Book | 2022 | §5.14 | 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 | |
| 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 (C) | 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
Every coating sits on the surface and wears away with use; coatings are inherently non-permanent. Mystic and Azotic topaz, CVD film on diamond, polymer-coated pearl and resin-coated opal are the usual cases; what the film is made of is not something the eye reports.
What to ask for on the report
An invoice that says Mystic or Azotic has named a brand, not a substance. The naming pattern reserves a slot for the substance, so ask which material the film is made of.
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.
- Every coating sits on the surface and wears away with use; coatings are inherently non-permanent. Mystic and Azotic topaz, CVD film on diamond, polymer-coated pearl and resin-coated opal are the usual cases; what the film is made of is not something the eye reports.
- An invoice that says Mystic or Azotic has named a brand, not a substance. The naming pattern reserves a slot for the substance, so ask which material the film is made of.
- Must be disclosed. Coatings may be abraded, flake off or discolour, and coating is not a permanent enhancement; the coating material must be disclosed (e.g. Mystic Topaz is coated topaz), and the treated effect must not be implied to be natural.
Questions about this treatment?
Disclosure practice for this treatment differs between authorities and markets, and a report does not always say what was done.