On this page
UnderstandWhat it is, what people ask, how far this page goes
Natural inorganic gem materials · Trade name: colour / appearance variety · Natural identity; treatment status unknown
Black Agate
A commercial colour name for dark chalcedony or agate; much of it on the market is dyed, so it should not be described as naturally black by default. This page compares how the major standards and institutions name it, what must be disclosed when selling it, whether it can be worn every day, how it is treated and identified, and which source each conclusion comes from.
Price, rarity and origin are outside the scope of this wiki.
What this page can answer
This page covers
- How Black Agate forms and what gives its colourFormation setting, process and timescale, each finding graded and sourced
- How to identify Black AgateIdentification points from the research literature; how far lab testing can go
- Treatments and enhancements of Black AgateThe enhancements and treatments the GB/T and JJA/AGL tables list
- Caring for and wearing Black AgateCleaning, heat, chemicals and light, as the cited sources give them
- Hardness, density and refractive index of Black AgateEach value with its source and check date
- Black Agate in English, Chinese and Japanese; national standard namesNames in four languages, the GB/T standard name and the suggested listing name; and: Has any of GIA, FTC, CIBJO or Gem-A put a name for it on record here?
- Listing and disclosing Black AgateThe listing name to use; what must be disclosed before sale
What we cannot conclude
- Whether one particular bead is natural, treated or synthetic — only a laboratory report can settle that.
- Geographic origin, unless an origin report accompanies the item.
- Price, rarity or investment value: this wiki covers naming and disclosure only.
Material research: formation, causes and history
What authoritative sources say about how this material forms, what causes its colour and optical effects, where its name comes from and how people have used it. Each statement carries an evidence grade and its sources; it describes the material in general, not any specific piece.
How it forms
Black agate is a trade colour name for dark (black) chalcedony/agate, whose dark colour commonly comes from dyeing in the market rather than from nature; black should therefore be treated as a treatment feature to be identified, not assumed natural. If natural and banded, its genesis follows agate: voids in the rock are filled cyclically from the rim toward the centre by silica-rich fluids, the silica precipitating layer by layer as fibrous microcrystalline quartz to form bands; band number, width and pattern depend on silica concentration, temperature, pressure and the timing of fluid influxes, while Fe, Ti and Mn affect only colour. Dark colour may also arise naturally from impurities or later hydrothermal processes. Chalcedony-type material can be coloured with many dyes, and in dyed blue chalcedony the copper concentrates in the outer layer and along fractures, which is strong evidence of dyeing.
Scope: Used to explain that "black agate" is a trade colour name and that its black colour is commonly the result of dyeing rather than natural formation, and to note the need to identify it as treated material.
Sources G&G Spring 2020 · G&G Summer 2020 · G&G Fall 2013
- Dyed Chalcedony Imitation of Chrysocolla-in-Chalcedony (Gems & Gemology, Spring 2020, Vol. 56, No. 1)
- Quench-Crackled Dyed Blue Chalcedony Resembling Larimar (Gems & Gemology, Summer 2020, Vol. 56, No. 2, Lab Notes)
- Agates from Morocco: Gemological Characteristics and Proposed Origin (Gems & Gemology, Fall 2013, Vol. 49, No. 3)
Where it forms
Voids and vesicles in volcanic rock (geodes, vugs) and fissures in sedimentary rocks, where silica-rich fluids fill them cyclically to form banded chalcedony.
Sources G&G Fall 2013 · G&G Spring 2020 · G&G Summer 2020
- Agates from Morocco: Gemological Characteristics and Proposed Origin (Gems & Gemology, Fall 2013, Vol. 49, No. 3)
- Dyed Chalcedony Imitation of Chrysocolla-in-Chalcedony (Gems & Gemology, Spring 2020, Vol. 56, No. 1)
- Quench-Crackled Dyed Blue Chalcedony Resembling Larimar (Gems & Gemology, Summer 2020, Vol. 56, No. 2, Lab Notes)
Post-formation treatment setting (commercial): dye baths (e.g. copper salts) penetrate the outer layer and fractures.
Sources G&G Spring 2020 · G&G Summer 2020
How it forms, step by step
(If natural banded chalcedony) silica-rich fluids fill a rock void cyclically from rim to centre, precipitating fibrous microcrystalline quartz layer by layer.
Sources G&G Fall 2013 · G&G Summer 2020 · G&G Spring 2020
- Agates from Morocco: Gemological Characteristics and Proposed Origin (Gems & Gemology, Fall 2013, Vol. 49, No. 3)
- Quench-Crackled Dyed Blue Chalcedony Resembling Larimar (Gems & Gemology, Summer 2020, Vol. 56, No. 2, Lab Notes)
- Dyed Chalcedony Imitation of Chrysocolla-in-Chalcedony (Gems & Gemology, Spring 2020, Vol. 56, No. 1)
Fe, Ti, Mn and Fe compounds colour the bands; dark colour may also arise from impurities or later hydrothermal processes.
Commercial processing: the material is soaked in a dye bath, and dye (e.g. copper salts) penetrates the outer layer and fractures, concentrating colour at the surface and decreasing inward.
Dyeing along fractures can produce 'quench-crackled dyed' features useful for identifying treatment.
Link to appearance
Chalcedony/agate that is black or near-black; a uniform deep black often suggests dyeing, which can concentrate colour in the outer layer and mark fractures as networks.
Sources G&G Spring 2020 · G&G Summer 2020
Timescale
Natural chalcedony crystallization took millions of years (Sidi Rahal case); dyeing is a rapid artificial treatment outside the natural timescale.
Sources G&G Fall 2013 · G&G Spring 2020 · G&G Summer 2020
- Agates from Morocco: Gemological Characteristics and Proposed Origin (Gems & Gemology, Fall 2013, Vol. 49, No. 3)
- Dyed Chalcedony Imitation of Chrysocolla-in-Chalcedony (Gems & Gemology, Spring 2020, Vol. 56, No. 1)
- Quench-Crackled Dyed Blue Chalcedony Resembling Larimar (Gems & Gemology, Summer 2020, Vol. 56, No. 2, Lab Notes)
Related base materials
- Agate: banded form | Black agate is a commercial colour name for dark chalcedony (agate), which may be banded
- Chalcedony: mineral-base | Composed of microcrystalline quartz (chalcedony)
Supporting findings
If black agate is natural banded chalcedony, it formed as silica-rich fluids filled a rock void cyclically from rim to centre, precipitating layered bands.
Evidence grade AScope: Describes natural banded chalcedony; does not apply to purely dyed material.
How it is identified
For black agate, colour and banding by themselves do not reveal whether the hue is natural or produced by dyeing or heating, so visual appearance cannot settle naturalness or treatment; a trustworthy identification and any treatment ruling call for instrumental examination at a qualified gem laboratory, and this entry is no basis for self-testing.
Evidence grade AScope: Appearance cannot replace identification; naturalness, dyeing or heat treatment must be determined at a qualified gem laboratory, and this entry is not a self-testing basis.
Sources G&G Summer 2014 · CIBJO Gemstone Book 2022
Other research findings
Black agate is a trade colour name for black chalcedony/agate; its uniform deep black colour in the market commonly comes from dyeing rather than nature.
Evidence grade BScope: Sources document that chalcedony can be dyed with many dyes and give blue examples; the specific black-dyeing process is not directly documented.
Dyeing concentrates the colorant in the outer layer and along fractures, with colour decreasing from surface to interior — strong evidence of dyeing (including black).
Evidence grade AScope: The evidence comes from Cu-concentration mapping of copper-dyed blue chalcedony; extension to other colours requires verification.
As chalcedony, black agate has a refractive index of about 1.53–1.54 and specific gravity of about 2.48–2.65, composed mainly of quartz (possibly with moganite).
Evidence grade AScope: Laboratory ranges for chalcedony; dyeing does not substantially change these basic properties.
Limits of this research
- Research status: Formation, occurrence or other material-specific findings found in authoritative sources
- 'Black agate' is a trade colour name, and the 'commonly dyed' assessment rests on the general fact that chalcedony is dyed; the specific black-dyeing practice is not documented in the cited sources.
- The colouring mechanism of naturally black chalcedony (impurities, radiation, etc.) is not covered by the cited sources.
Methodology
- Origin determination and its limits
- Quality grading and value factors
- How gemmology became a technical discipline
Evidence grades
- A
- Standards bodies, government geological surveys, peer-reviewed research and museum records
- B
- Institutional educational material
- C
- Specialist databases or cautious readings
A statement is never graded above its strongest source.
This section is LumaLark's own factual restatement of public authoritative sources, credited item by item; it contains no verbatim excerpts, images or third-party logos.
DecideListing name, disclosure, care and treatments
- Suggested listing name
- Black Agate (may be dyed)
GB/T 16552—2017 5.1.1 c)
Construction or treatment status
Natural identity; treatment status unknown
Treatments recorded for this material: Heated / Heat-treated (via parent material), Filled / Fracture-filled (via parent material), Dyed (via parent material)
Related:Disclosure and cautions
This page cannot answer
Natural? · Origin · Price
A lab report settles these; this wiki covers naming and disclosure.
Related:What this page can answer
Answer at a glance
"Black Agate" is a market name rather than a mineral species name; before using it, verify: Trade name rather than a mineralogical species name.
What this means for you
Before buying, check the recommended listing name "Black Agate (may be dyed)" and any disclosed treatments on this page, and ask the seller for a grading report if in doubt.
Known uncertainty
- This entry's exact material identity is not fully settled.
- See “Disclosure and cautions” above
- Last reviewed
- 2026-09-01
Trade-name ruling
- Trade name rather than a mineralogical species name.
- Suggested listing name
- Black Agate (may be dyed)
Care and wear
Care
Black Agate — care (special care): dyed black (onyx): normal care.
AGTA Gemstone Information Manual, 17th ed.
- Treatment state
- treated
- Only after these treatments
- dyeing
Source & check 2026-09-26
- Source
- AGTA Gemstone Information Manual, 17th edition (August 2026) (AGTA): p.VIII, Gemstone Information Chart, Chalcedony Black (Onyx) D, CARE REQUIRED / SPECIAL ADVICE (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Each row's “Source & check” expands to the full source; the date beside it is the fact check date.
Treatments and identification leads
One row per treatment, with three standards side by side: how often it occurs and how stable it is (CIBJO), its category in GB/T 16552—2017 Table B.1 (this site's paraphrase) and how the JJA/AGL 2025 table lists it. Each CIBJO value links to its entry under “Testing and market”, where the wording and source are given in full.
This table scrolls sideways on narrow screens.
| Treatment | Market frequency (CIBJO) | Stability (CIBJO) | GB/T 16552 category | JJA/AGL listing | Identification leads |
|---|---|---|---|---|---|
| Heating | — | — | Optimization | — | — |
| Filling | — | — | Graded — see §5.4.1, Table 1 | — | — |
| Dyeing | — | — | Optimization | — | — |
“—” means the cited source gives no value for that item, not “none”. Source: GB/T 16552—2017 Table B.1 (row for Chalcedony)
Whether a treatment has to show in the name differs between the two rule sets (this site’s paraphrase):
- GB/T 16552—2017: a method classed as treatment has to be stated in the name itself — the method put before the name, or added in brackets after it; a method classed as optimization does not need to appear in the name, though one marked “Optimization (annotation required)” still has to be noted in the quality documents. Source: §5.4.2.1 a), b); §5.4.2.2 a)
- CIBJO Gemstone Book 2022: no longer splits treatments into ones that need specific disclosure and ones that need only general disclosure: every treatment is to be disclosed, and the seller tells the buyer which treatment was used and what special care it calls for. Source: §4.2.5 Note 2, §4.2.5.1, pp.14–15
What this table covers
What this page can give
- Registered treatments, their stability and how often they occur (see the table above)
- Identification leads for each treatment, for use when sending a stone for testing
VerifyNames, standards, technical facts, clauses and sources
Names
Besides Black Agate, the name register records "Black agate", for which no restriction on use as a name is recorded.
Standard and listing namesThe Chinese standard naming counterpart for Black Agate is agate (GB/T 16552—2017), but this name itself is not listed in the original; the suggested listing name is Black Agate (possibly dyed). Names in four languagesFour-language names are given in Simplified Chinese, Traditional Chinese, English (Black Agate) and Japanese; the Japanese name is the entry's own, and the wording of Japan's GSJ for it is not yet verified by this site. Trade and mineralogical namesThe mineralogical description is dark chalcedony or agate, commonly dyed, and the trade name/synonym is Black agate. Institution usageOn authority wording, the FTC and CIBJO texts on file have no wording for Black Agate (checked), and GIA and Gem-A are unchecked; bodies without a recorded wording show the entry's English name Black Agate, which does not represent their position. Sub-names and unacceptable namesThe Chinese name for black agate is flagged as a related high-risk name because black material on the market is commonly dyed and must be disclosed.
Name register
Every name recorded for this entry, with how it is classified, whether it may be matched to this entry automatically, whether it may be used as a product name, and the clause it rests on.
This table scrolls sideways on narrow screens.
| Name | Type | Matching | Use as a name | Basis |
|---|---|---|---|---|
| no restriction recorded (1) | ||||
| Black agateEnglish | market name | matched automatically | no restriction recorded | editorial ruling (no clause cited); Note: Related high-risk name “Black Onyx”: black material on the market is often dyed and must be disclosed. |
How each authority words this name: see Names › Authority wording.
- GSJ (Japan) formal name
- ブラックアゲート (not yet checked for this body — the entry's own Japanese name is shown)
- GB/T naming counterpart (this name is not in the standard)
- 玛瑙 (GB/T 16552—2017 §5.1.1 c))
- Mineralogical name
- Dark chalcedony/agate, commonly dyed
Four-language names
- 简体中文
- 黑玛瑙
- 繁體中文
- 黑瑪瑙
- English
- Black Agate
- 日本語
- ブラックアゲート
See also: Misleading and prohibited trade names · Supplementary name index
Classification
- Origin
- Natural materials
- Material domain
- Natural inorganic gem materials
- Browsing family
- Quartz and chalcedony materials
- Identity tier
- Trade name: colour / appearance variety
- Mineral chemical class
- Silicates
- Construction or treatment status
- Natural identity; treatment status unknown
Treatments listed by standards
The treatment classification of Black Agate is in GB/T 16552—2017 Annex B, Table B.1, applied by this site from the "Chalcedony (Agate/Jasper)" row: heating is an optimization, filling is graded, and dyeing is an optimization. The full row with the effect column is on the Chalcedony entry; what Table B.1 is, and how the filling grades are defined, are on this site's Gem treatments page.
GB/T 16552-2017, Annex B, Table B.1
Under the “Chalcedony (Agate/Jasper)” row of Table B.1: Heating Optimization; Filling Graded — see §5.4.1, Table 1; Dyeing Optimization. The full row, with the effect column, is on the Chalcedony page.
What Table B.1 is, and how the “see 5.4.1” grades are defined: see the GB/T 16552: optimization vs treatment page.
What it is and how it's identified
The registered material facts for Black Agate cite clauses written for chalcedony, including agate and jasper (the whole class); they are not measurements of Black Agate itself. Whole-class features such as chemical composition, luminescence, UV-visible spectrum, infrared spectrum and magnification are in the material facts on the Chalcedony entry. As registered under the clauses for chalcedony, including agate and jasper (the whole class): Black Agate has a cryptocrystalline aggregate structure, compact and massive, and may also form spherulitic, radiating or microfibrous aggregates; Mohs hardness 5–7; cleavage none; density 2.5–2.77 g/cm³; optic character anisotropic aggregate; refractive index 1.535–1.539, which this site notes is the overall value although the clause has finer sub-ranges; birefringence not measurable for an aggregate; untreated Black Agate has pleochroism not measurable for an aggregate; untreated Black Agate has the quality factor (lustre) vitreous to greasy; under the chalcedony class clause, the optical effects iridescence and chatoyancy are both recorded as visible.
Registered material facts
Each row carries its own source and check date and can be cited on its own; “—” means the cited source gives no value, not “none”.
Chemical composition, luminescence, UV-visible spectrum, infrared spectrum, magnification: these whole-class features are given in the cited clause for chalcedony, including agate and jasper (the whole class), so they are not repeated on the Black Agate page; see the material facts on the Chalcedony entry.
Items below that name the class they cover give the cited clause’s value for that whole class, not a measurement of Black Agate (how to read class-level values).
Structure
Black Agate — structure (overall): a cryptocrystalline aggregate, dense and massive in habit; it may also form spherulitic, radial or finely fibrous aggregates.
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Durability
Black Agate — Mohs hardness: 5–7
GB/T 16553—2017
Reference points on this site: Talc 1–3, Fluorite 4, Apatite 5–5.5, Orthoclase 6–6.5, Topaz 8, Diamond 10 (Source: GB/T 16553—2017); Gypsum 1.5–2, Calcite 3, Quartz 7 (Source: Handbook of Mineralogy); Corundum 9 (Source: GIA Gem Encyclopedia (care guide))
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Black Agate — cleavage: none
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Physical properties
Black Agate — density: 2.5–2.77 g/cm³
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Optical
Black Agate — optic character: anisotropic aggregate
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Black Agate — refractive index: 1.535–1.539
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
- General note
- the clause has finer ranges
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Black Agate — birefringence: not measurable on an aggregate
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Black Agate — pleochroism: not measurable on an aggregate
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Black Agate — optical effect (iridescence): seen
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Black Agate — optical effect (chatoyancy): seen
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
- CIBJO definitions
- chatoyancy
Quality factors
Black Agate — quality factor (lustre): vitreous to greasy
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- What this covers
- chalcedony, including agate and jasper (the whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.5 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Each row's “Source & check” expands to the full source; the date beside it is the fact check date.
This material (RI / SG midpoints): 1.537 / 2.63; Closest values: 1Amethyst 1.548 / 2.67, 2Charoite 1.554 / 2.66, 3Changhua Stone 1.56 / 2.6, 4Shoushan Stone 1.56 / 2.7, 5Balin Stone 1.56 / 2.55
Show the full chart (330 registered materials)
Outside the full chart's range, drawn as triangles at the edge (RI / SG midpoints): Hematite 3.045/5.12, Synthetic Rutile 2.76/4.26, Rutile 2.753/4.23, Synthetic Moissanite 2.67/3.22
Testing and market
The CIBJO laboratory protocol covers Black Agate under “Chalcedony” (§6.1.16): see identification on the Chalcedony entry; the required tests are listed on the CIBJO laboratory protocol table for Chalcedony.
How far testing can go
Each row's “Source & check” expands to the full source; the date beside it is the fact check date.
Evidence and sources
Authority basis and clause sources
Sources
Further reading
Related entries and topic cluster
Parent entry and related pages on this site. Rows marked “Planned” are suggested new content and have no link yet.
Related-entry tree for Quartz on this site: 74 entries listed by this site's parent-entry links; synthetic, treated and similar entries carry their recorded state after the name; this page (Black Agate) is marked.
- Quartz
Sub-entries (57)
- Agate
Sub-entries (1)
- Amethyst
- Ametrine
- Aventurine Quartz
- Aventurine Quartz / Green Aventurine
- Banded Agate
- Black Agate
- Bloodstone / Heliotrope
- Blue Lace Agate
- Carnelian
- Chalcedony
Sub-entries (2)
- Chrysoprase
- Citrine
- Clear Quartz / Rock Crystal
- Coated Quartz
- Colourless to White Quartz (brand name: Azeztulite)
- Crazy Lace Agate
- Dendritic Agate
- Dyed Agate (Treated: dyed)
- Elestial Quartz / Skeletal Quartz
- Eye Agate
- Flower Agate / Cherry Blossom Agate
- Green Quartz
- High-clarity quartz with golden needle-like inclusions
- Included Quartz
- Jasper
Sub-entries (1)
- Madeira Citrine
- Malachite Agate
- Morion / very dark Smoky Quartz
- Moss Agate
- Moss Chalcedony
- Multi-inclusion Quartz
- Onyx (banded chalcedony)
- Phantom Quartz
- Prase
- Prasiolite / Green Quartz
- Quartz Cat’s-eye
- Quartz claimed to be from the Himalayas
- Quartz Crystal
- Quartz with dark needle-like inclusions
- Quartz with fine fibrous inclusions
- Quartz with flower-like inclusions
- Quartz with golden needle-like inclusions
- Quartz with green needle-like inclusions
- Quartzite Jade
- Quartzose Jade
- Red Agate
- Rock Crystal
- Rose Quartz
- Rutilated Quartz
- Sard
- Sardonyx
- Silicified Asbestos
Sub-entries (2)
- Hawk’s Eye
- Tiger’s Eye
Sub-entries (1)
- Smoky Quartz
- Smoky Quartz (brown variety)
- Strawberry Quartz
- Yellow-green Quartz
- Agate
This table scrolls sideways on narrow screens.
| Page | Content | Status |
|---|---|---|
| Browsing family | Quartz and chalcedony materials | Live |
| Treatments recorded for this material | Heated / Heat-treated (via parent material), Filled / Fracture-filled (via parent material), Dyed (via parent material) | Live |
| Related articles | To be added (placeholder) | Planned |
Who maintains this page
The material facts in the Black Agate entry are maintained by the LumaLark wiki and matched one by one to the standard clauses or institutional sources they cite: every fact carries its source (with a clause or page locator) and a check date, so it can be traced back to the original. The entry exists so that buyers and sellers can check naming, disclosure and material facts item by item.