On this page
UnderstandWhat it is, what people ask, how far this page goes
Natural organic and biogenic materials · Organic or biogenic material · Natural identity; treatment status unknown
Sponge Coral
Also known asCongi · Foam coral · red spongy coral
A porous, non-precious coral material that is almost always dyed and/or resin-stabilized; it must not be called Precious Coral or natural-color coral. Also known as Foam coral, Congi. 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 Sponge Coral forms and what gives its colourFormation setting, process and timescale, each finding graded and sourced
- How to identify Sponge CoralIdentification points from the research literature; how far lab testing can go
- Treatments and enhancements of Sponge CoralThe enhancements and treatments the GB/T and JJA/AGL tables list
- Caring for and wearing Sponge CoralCleaning, heat, chemicals and light, as the cited sources give them
- Hardness, density and refractive index of Sponge CoralEach value with its source and check date
- Sponge Coral 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 Sponge CoralThe 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
Sponge coral is a common coral: species of the family Melithaeidae (e.g. Melithaea ochracaea), named for its highly porous skeleton that resembles a sponge. It is not a precious coral (it does not belong to the Corallidae). Because of its many holes, sponge coral was until recently little used for jewellery; to be used today it relies heavily on stabilisation — impregnation and filling with resins or polymers, followed by polishing. In addition to filling, part of the material is dyed, and a little has, by report, been 'pressed' (crushed) and mixed with epoxy so as to be formed into shapes. It is commonly marketed as natural Congi or as 'red spongy coral'. Coral itself is skeleton-bearing marine invertebrate of the phylum Cnidaria, formed by nature without human intervention.
Scope: Applies to an overview of the naming boundary of sponge coral as a common coral of the family Melithaeidae (not precious coral), its porous skeleton, stabilisation / dyeing treatments and trade names; specific identification and treatment disclosure must be handled case by case.
Sources CIBJO Coral Blue Book 2024
Where it forms
Benthic marine settings; sponge coral belongs to the common coral family Melithaeidae.
Sources CIBJO Coral Blue Book 2024
How it forms, step by step
The sponge-coral colony grows on the seabed, forming a highly porous skeleton.
Sources CIBJO Coral Blue Book 2024
Because of the many holes, the material must be impregnated and filled with resin or polymer (stabilised) and polished before use in jewellery.
Sources CIBJO Coral Blue Book 2024
Besides being filled, part of the material is dyed, and a little is crushed and blended with epoxy before being moulded into shapes.
Sources CIBJO Coral Blue Book 2024
Link to appearance
Sponge coral has a highly porous, sponge-like appearance and can only be polished after stabilisation; filling and dyeing alter its appearance and stability.
Sources CIBJO Coral Blue Book 2024
Timescale
The source gives no absolute timescale for sponge-coral formation or growth.
Sources CIBJO Coral Blue Book 2024
Related base materials
- Common coral (family Melithaeidae): Sponge coral is a common coral of the family Melithaeidae and is not a precious coral
Supporting findings
Coral is skeleton-bearing marine invertebrate of the phylum Cnidaria, formed by nature without human intervention.
Evidence grade AScope: A material-classification statement.
Sources CIBJO Coral Blue Book 2024
Recorded localities
This is an organic material; its localities are the harvest or culture areas (waters or species ranges) recorded by authoritative sources.
Seas and coasts
- Waters off Taiwan, China
Sources CIBJO Coral Information Sheet
- Waters off Indonesia
Sources CIBJO Coral Information Sheet
These are localities recorded by authoritative sources, not an origin finding for any individual piece. The CIBJO Gemstone Book (2022, §4.2.3) says a place of origin names where a gem was mined or harvested, is a matter of opinion, does not imply a level of quality, and that cutting, processing or exporting centres must not be used to imply origin. See how far testing can go on this page, the origin determination guide and the overview of recorded localities.
How it is identified
Sponge coral is porous and resembles bone, other corals, and imitations, so unaided inspection struggles to establish its material identity or treatment status. Confirmation relies on laboratory testing such as microscopic structural examination and spectroscopic instruments; the cited sources give no self-test criteria, and conclusions must be issued by an accredited laboratory.
Evidence grade AScope: Appearance cannot replace identification; testing by a qualified laboratory is required, and this entry gives no self-test criteria or threshold.
Sources CIBJO Coral Blue Book 2024
Other research findings
Sponge coral is a common coral of the family Melithaeidae (e.g. Melithaea ochracaea), named for its highly porous, sponge-like skeleton, and is not a precious coral.
Evidence grade AScope: Naming and classification; explicitly a common coral.
Sources CIBJO Coral Blue Book 2024
Sponge coral has a highly porous skeleton and must be impregnated and filled with resin or polymer (stabilised) and then polished before use in jewellery.
Evidence grade AScope: Describes skeleton porosity and the stabilisation requirement.
Sources CIBJO Coral Blue Book 2024
Sponge coral is frequently filled and dyed; a little of it has, by report, been crushed and blended with epoxy to be moulded into shapes; it is commonly marketed as natural Congi or as 'red spongy coral'.
Evidence grade AScope: Treatment and trade-name description; treatment must be disclosed.
Sources CIBJO Coral Blue Book 2024
Because sponge coral relies heavily on treatment, its treatment and naming must be disclosed carefully (CIBJO sets disclosure requirements for common and treated corals).
Evidence grade BScope: A disclosure-and-naming statement; see the relevant CIBJO coral clauses.
Sources CIBJO Coral Blue Book 2024
Limits of this research
- Research status: Formation, occurrence or other material-specific findings found in authoritative sources
- Single source (the CIBJO coral standard), focused on trade classification and nomenclature; it does not describe the physiology of skeleton deposition.
- The database flags this name as requiring identification and compliance review: sponge coral relies heavily on stabilisation/dyeing, so identification and disclosure must be handled case by case.
- The source gives no absolute timescale for sponge-coral formation/growth.
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
- Sponge Coral (state any treatments)
16 CFR Part 23 §23.1 · CIBJO Coral Blue Book 2024 §5.28
Construction or treatment status
Natural identity; treatment status unknown
Treatments recorded for this material: Bleached (via parent material), Filled / Fracture-filled (via parent material), Coated (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
"Sponge Coral" is a market name rather than a mineral species name; before using it, verify: Laboratory identification required before the name is used.
Evidence
What this means for you
Before buying, check the recommended listing name "Sponge Coral (state any treatments)" 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
- Laboratory identification required before the name is used.
- Suggested listing name
- Sponge Coral (state any treatments)
Care and wear
The normal care that applies to all corals, and so to Sponge Coral (CIBJO Coral Book §6.1), and the special care the source gives for some corals without saying which ones (§6.2), are set out in full under care on the Coral entry.
Care
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 |
|---|---|---|---|---|---|
| Bleaching | — | — | Optimization | — | — |
| Filling | — | — | Graded — see §5.4.1, Table 1 | — | — |
| Coating | — | — | Graded — see §5.4.1, Table 1 | — | — |
| Dyeing | — | — | Treatment | — | — |
“—” means the cited source gives no value for that item, not “none”. Source: GB/T 16552—2017 Table B.1 (row for Coral)
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 Sponge Coral, the name register records "Congi", "Foam coral" and "red spongy coral", for which no restriction on use as a name is recorded (CIBJO Coral Blue Book 2024 Annex C 7.9.4 “Sponge coral” “Congi / red spongy coral”).
Standard and listing namesThe GB/T 16552—2017 naming counterpart for Sponge Coral is coral (Table A.3), and the name itself is not listed in the standard; the suggested listing name is Sponge Coral (stating treatment). Trade and mineralogical namesThe English description is a porous non-precious coral material, often dyed or stabilised; the trade name or synonym is Foam coral. Institution usageIn the name register, the CIBJO Coral Blue Book 2024 registers Congi and red spongy coral as trade names for “Sponge coral”, Foam coral is a market name ruled on by this site (no clause attached), and the related high-risk name is “Coral / Precious Coral”. Names in four languagesNames are registered in four languages: Simplified Chinese, Traditional Chinese, English (Sponge Coral) and Japanese. Institution usageOn authority wording, the FTC text on file has no wording for this name (checked), and the wording of GIA, CIBJO and Gem-A is unchecked; bodies without a recorded wording show the entry's own English name, Sponge Coral, which does not represent their position. Japanese namesThe Japanese name shown is the entry's own; the wording used by the GSJ (Japan) for this name is not yet verified by this site.
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 (3) | ||||
| CongiEnglish | market name | matched automatically | no restriction recorded | CIBJO Coral Blue Book 2024 Annex C 7.9.4 “Sponge coral” “Congi / red spongy coral” |
| Foam coralEnglish | market name | matched automatically | no restriction recorded | editorial ruling (no clause cited); Note: Related high-risk name “Coral / Precious Coral”. |
| red spongy coralEnglish | market name | matched automatically | no restriction recorded | CIBJO Coral Blue Book 2024 Annex C 7.9.4 “Sponge coral” “Congi / red spongy coral” |
How each authority words this name: see Names › Authority wording.
- GSJ (Japan) formal name
- 海綿珊瑚 (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)
- 珊瑚 (GB/T 16552—2017 Table A.3)
- Mineralogical name
- Porous coral material from non-precious coral species; often dyed or stabilised
- Trade names and synonyms
- Foam coral (market name) · Congi (market name) · red spongy coral (market name)
Four-language names
- 简体中文
- 海绵珊瑚
- 繁體中文
- 海綿珊瑚
- English
- Sponge Coral
- 日本語
- 海綿珊瑚
See also: Misleading and prohibited trade names · Supplementary name index
Classification
- Origin
- Natural materials
- Material domain
- Natural organic and biogenic materials
- Browsing family
- Coral materials
- Identity tier
- Organic or biogenic material
- Mineral chemical class
- Not applicable
- Construction or treatment status
- Natural identity; treatment status unknown
Treatments listed by standards
Sponge Coral is not listed separately in GB/T 16552—2017, Annex B, Table B.1; this site applies the table's “Coral” entry: bleaching is an optimization; filling and coating both need to be graded; dyeing is a treatment. The effect of each is described on the Coral entry, and what Table B.1 is and how the grades are defined are on the Gem treatments page.
GB/T 16552-2017, Annex B, Table B.1
Under the “Coral” row of Table B.1: Bleaching Optimization; Filling Graded — see §5.4.1, Table 1; Coating Graded — see §5.4.1, Table 1; Dyeing Treatment. The full row, with the effect column, is on the Coral 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
Sponge Coral is a common coral of the family Melithaeidae, porous, and classed as calcareous coral. Most material facts take the cited clauses' values for a class, not measurements of this variety itself. For coral as a whole (calcareous and horny): cleavage usually not seen in the aggregate, optic character aggregate, and, untreated, pleochroism not measurable on an aggregate and a waxy lustre, vitreous on polished surfaces. For calcareous coral: Mohs hardness 3–4.5, with a site note that this is the class range, not a value measured on this variety; density 2.6–2.7 g/cm³, with a site note that it is worked out from the clause value 2.65 and tolerance of ±0.05; refractive index 1.48–1.66, with a site note that this is the reading commonly obtained with the spot method, a typical value rather than an absolute limit; untreated, it effervesces with a drop of hydrochloric acid.
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 calcareous coral, coral as a whole (calcareous and horny), so they are not repeated on the Sponge Coral page; see the material facts on the Coral entry.
Items below that name the class they cover give the cited clause’s value for that whole class, not a measurement of Sponge Coral (how to read class-level values).
Definition
Sponge Coral: a common coral (5.25) belonging to the family Melithaeidae of the order Alcyonnacea. Its name is derived from its similar appearance to sponges. Until recently sponge coral was not used for jewellery because it has too many holes.
CIBJO Coral Blue Book 2024
- Qualification
For use in jewellery it almost always has to be stabilised first by filling with resin or polymer; some material is also dyed.
Source & check 2026-09-22
- Source
- CIBJO Blue Book — Coral Book (Coral Commission 2024) (CIBJO): §7.9.4 (Annex B, normative), p.28 (locator checked 2026-09-22)
- Fact checked on
- 2026-09-22
Structure
Sponge Coral — structure (overall): calcareous coral: the inorganic part is a cryptocrystalline aggregate and the organic part is non-crystalline. Horny coral: non-crystalline.
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- coral as a whole (calcareous and horny)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Durability
Sponge Coral — Mohs hardness: 3–4.5
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
- calcareous coral
- Qualification
This is the range for the whole class, not a measured value for this variety. The value comes from the calcareous branch of the cited clause; the CIBJO Coral Book lists this coral as calcareous.
Source & check 2026-09-20
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-20)
- Fact checked on
- 2026-09-20
Sponge Coral — cleavage: generally not seen on an aggregate
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- coral as a whole (calcareous and horny)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Sponge Coral — stability: effervesces on contact with hydrochloric acid
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- What this covers
- calcareous coral
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Physical properties
Sponge Coral — density: 2.6–2.7 g/cm³
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- calcareous coral
- Qualification
The clause we cite gives 2.65, with a stated tolerance of ±0.05. The range shown here is worked out from that.
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Optical
Sponge Coral — optic character: aggregate
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- coral as a whole (calcareous and horny)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Sponge Coral — refractive index: 1.48–1.66
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- calcareous coral
- Qualification
The clause we cite gives this as a value commonly read by the spot (distant-vision) method. It is a common value, not an absolute limit.
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Sponge Coral — pleochroism: not measurable on an aggregate
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- What this covers
- coral as a whole (calcareous and horny)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Quality factors
Sponge Coral — quality factor (lustre): waxy; polished surfaces take a vitreous lustre.
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- What this covers
- coral as a whole (calcareous and horny)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.3.4 (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): 1.57 / 2.65; Closest values: 1Scapolite 1.558 / 2.67, 2Changhua Stone 1.56 / 2.6, 3Amethyst 1.548 / 2.67, 4Balin Stone 1.56 / 2.55, 5Qingtian Stone 1.565 / 2.77
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
Related CIBJO terms
- Sponge coral
common coral belonging to Melithaeidae family (e.g. Melithaea ochracaea) with characteristic highly porous skeleton requiring stabilisation treatment with impregnation and filling with resins or polymers before being polished. Sometimes dyed or assembled with epoxy.
CIBJO Blue Book — Coral Book (Coral Commission 2024) (CIBJO): §5.92, p.21
Definitions are quoted verbatim from the cited CIBJO Blue Book; […] marks an excerpt, and the full definition is at the cited clause.
Testing and market
The CIBJO laboratory protocol covers Sponge Coral under “Coral” (§6.2.4): see identification on the Coral entry; the required tests are listed on the CIBJO laboratory protocol table for Coral.
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
- CIBJO – The World Jewellery Confederation. Gemmological Laboratories Book (Blue Book), Gemmological Commission 2024-1.
- CIBJO – The World Jewellery Confederation. The Gemstone Book (Blue Book), 2022.
- SAMR / SAC (China). GB/T 16552—2017 Gems — Nomenclature.
- GIA – Gemological Institute of America (United States). Gem Encyclopedia.
- Gem-A – The Gemmological Association of Great Britain (United Kingdom).
- eCFR (United States). 16 CFR § 23.1 — Deception (general). In: 16 CFR Part 23 — Guides for the Jewelry, Precious Metals, and Pewter Industries.
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 Coral on this site: 12 entries listed by this site's parent-entry links; synthetic, treated and similar entries carry their recorded state after the name; this page (Sponge Coral) is marked.
This table scrolls sideways on narrow screens.
| Page | Content | Status |
|---|---|---|
| Browsing family | Coral materials | Live |
| Treatments recorded for this material | Bleached (via parent material), Filled / Fracture-filled (via parent material), Coated (via parent material), Dyed (via parent material) | Live |
| Related articles | To be added (placeholder) | Planned |
Who maintains this page
The material facts in the Sponge Coral 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.