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UnderstandWhat it is, what people ask, how far this page goes
Natural inorganic gem materials · Mineral / gemstone variety · Natural identity; treatment status unknown
Opalised Shell
Also known asShell Opal
Material formed when fossil shell is replaced by opal; it must be distinguished from natural shell, Mother-of-Pearl / Nacre and opal assembled stones. 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 Opalised Shell forms and what gives its colourFormation setting, process and timescale; where its colour and optical effects come from, each finding graded and sourced
- How to identify Opalised ShellIdentification points from the research literature
- Caring for and wearing Opalised ShellCleaning, heat, chemicals and light, as the cited sources give them
- Hardness, density and refractive index of Opalised ShellEach value with its source and check date
- Opalised Shell in English, Chinese and Japanese; national standard namesNames in four languages, the GB/T standard name and the suggested listing name; and: Which of GIA, FTC, CIBJO and Gem-A have a name for it on record, and do those agree?
- Listing and disclosing Opalised ShellThe listing name to use; what must be disclosed before sale
- Standard clauses that define Opalised ShellWhich clause defines it, each cross-checked
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
Opalised shell is fossil material in which the shell of an invertebrate such as a mollusc has been replaced or permineralized by opal; it results from petrification, i.e. replacement. According to the NPS, permineralization is minerals filling the natural pores of original organic material, and it happens when groundwater carrying silica flows through tiny pores in buried wood, bone or shells; the term replacement, by contrast, denotes inorganic minerals substituting for the dissolved-away hard parts of organisms and involves a change of mineralogy; silicification—infilling and/or replacement by silica—is the commonest kind, and carbonate shells held by invertebrates—mollusks, brachiopods or crinoids among them—may also be silicified. The silica concerned can take several forms, among them 'Opal: hydrated amorphous silica'. Where opal is the silica that replaces or infills the shell, the result is opalised shell. Its parent-material difference from ordinary opal lies in this: ordinary opal forms when silica gel solidifies in cracks or layers of rock, whereas opalised shell keeps the shape of a biogenic shell while opal replaces its carbonate hard parts; play-of-color opal can also record the structural influence of a pre-existing fibrous mineral that opal has replaced.
Scope: Used to describe fossil material formed when a biogenic carbonate shell is replaced or infilled by opal; the parent body is a biological shell, distinguishing it from ordinary opal, in which silica gel solidifies directly in rock cavities.
Sources NPS Permineralization · G&G Spring 2017
Where it forms
Groundwater system in a burial environment: silica-bearing groundwater passing through the tiny pores of buried shells is the medium of replacement/infilling.
Sources NPS Permineralization
Volcanic-ash-rich sedimentary settings: volcanic ash is a ready source of silica that groundwater can carry to silicify fossils.
Sources NPS Permineralization
How it forms, step by step
After the biogenic carbonate shell is buried, silica-bearing groundwater passes through its tiny pores.
Sources NPS Permineralization
Silica either infills the shell's natural pores by permineralization, or substitutes for the dissolved-away carbonate hard parts by replacement.
Sources NPS Permineralization
When the silica involved takes the form of opal (hydrated amorphous silica), opalised shell is produced.
Sources NPS Permineralization
In some opal (e.g. from Ethiopia), a structure caused by a preexisting fibrous mineral replaced by opal may also be preserved.
Sources G&G Spring 2017
Link to appearance
Preserves the external form of the biogenic shell (a three-dimensional internal mold, sometimes with microscopic detail) while its mineralogy has been replaced by silica (opal); it may or may not show play-of-color.
Sources NPS Permineralization
Timescale
The cited sources give no numeric age for the replacement/petrification of opalised shell; they describe the process as occurring after burial and during diagenesis.
Sources NPS Permineralization
Related base materials
- Opal (hydrated amorphous silica): replacement-of | Opal replaces/fills the carbonate hard parts of the shell
- Biogenic carbonate shell: precursor-of | Shells (of molluscs/brachiopods/crinoids, etc.) are its precursor material, replaced by silica (opal)
Supporting findings
Replacement denotes the swapping of inorganic minerals for the hard parts of organisms once those parts have dissolved, with a consequent change of mineralogy; permineralization, by contrast, denotes minerals filling the natural pores of original organic material.
Evidence grade AScope: An NPS general definition of fossil-preservation processes; applies to fossils preserved from carbonate or woody hard parts.
Sources NPS Permineralization
Shells made of carbonate—those of invertebrates like mollusks, brachiopods or crinoids—are also capable of being silicified, that is, infilled and/or replaced by silica.
Evidence grade AScope: A general statement; it does not specifically discuss the name 'opalised shell'.
Sources NPS Permineralization
Play-of-color opal can preserve a structural influence from a preexisting fibrous mineral replaced by opal, indicating that opal can replace pre-existing minerals.
Evidence grade BScope: The source offers this as a possible cause ('could be due to…') of a flame/chatoyant structure, not a firm conclusion.
Sources G&G Spring 2017
Recorded localities
Authoritative sources list the localities below; the list is not exhaustive.
Oceania
- Coober Pedy, South Australia, Australia
Sources Mindat: Opal
- Coocoran opal fields, New South Wales, Australia
Sources Mindat: Opal
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 the origin determination guide and the overview of recorded localities.
Causes of colour and optical effects
Where an opalised shell retains play-of-color, the optics remain diffraction of white light by an orderly packing of nanoscale silica spheres; silicification infills and/or replaces the carbonate shell with silica—including opal in its hydrated amorphous form—and may preserve pre-existing structure such as a fibrous fabric, yet replacement does not necessarily build an ordered sphere grating, so not every opalised shell shows play-of-color.
Evidence grade AScope: The mechanism is the general principle of opal diffraction; replacement of pre-existing minerals by opal is drawn from observations on play-of-color opal from one source area (a proposed cause), and the claim neither asserts play-of-color in every opalised shell nor establishes locality, natural origin, or treatment.
Sources G&G Fall 2013 · G&G Spring 2017
How it is identified
Whether a shell-derived organic material is natural, treated or an imitation cannot be settled from its surface look. Laboratory identification of pearls and shell draws on standard gemological testing together with advanced instruments: real-time X-radiography, ultraviolet-visible reflectance spectroscopy, X-ray fluorescence, microradiography to gauge nacre thickness when needed, and computed micro-tomography for detailed work, with a second specialist independently repeating the observations. Because these methods and their interpretation belong to a gem laboratory, the identification should be carried out by a qualified institution.
Evidence grade AScope: Applies where the identity and treatment state of shell/pearl-type organic material must be judged; no self-administered criteria are given.
Sources GIA: How GIA Identifies Pearls · CIBJO Pearl Book 2024 · CIBJO Gemstone Book 2022
Other research findings
The silica involved in silicification can take several forms, including opal (hydrated amorphous silica).
Evidence grade AScope: The list is of silica forms in fossil silicification; it does not quantify how often opal is the replacing phase.
Sources NPS Permineralization
Limits of this research
- Research status: Formation, occurrence or other material-specific findings found in authoritative sources
- The NPS text describes fossil silicification/permineralization in general and does not specifically define the name 'opalised shell'.
- The evidence for 'replaced/infilled by opal' is that opal is among the silica forms involved; the source does not quantify how often opal is the replacing phase.
- The suggestion that a flame/chatoyant structure stems from a preexisting fibrous mineral replaced by opal is offered as a possibility in the source ('could be due to…').
- The cited sources give no specific numeric age.
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
- Opalised Shell
IMA Mineral List 2026-07 “Opal” · CIBJO Gemmological Laboratories Book 2024 §7.2.1.164 “Opalised shell”
Construction or treatment status
Natural identity; treatment status unknown
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
Must be distinguished from plain shell, from mother-of-pearl and from assembled opal; the palaeontological source of the specimen must be recorded.
Evidence
- CIBJO · 7.2.1.164
What this means for you
Before buying, check the recommended listing name "Opalised Shell" and any disclosed treatments on this page, and ask the seller for a grading report if in doubt.
Known uncertainty
- Last reviewed
- 2026-09-01
Care and wear
Normal care for every gemstone (CIBJO Gemstone Book §6.1), in brief:
- Handle it gently.
- Store pieces apart when not worn, so they cannot scratch each other.
- Clean with warm soapy water and a soft brush.
- Use ultrasonic cleaners only with caution.
The §6.1 wording and the full list of §6.2 special-care letters cited on this page are on the general gem care page.
Care
Each row's “Source & check” expands to the full source; the date beside it is the fact check date.
VerifyNames, standards, technical facts, clauses and sources
Names
Besides Opalised Shell, the name register records "Shell Opal", for which no restriction on use as a name is recorded (CIBJO Gemstone Book 2022 Annex 8 “Shell Opal”).
Standard and listing namesThe suggested listing name for Opalised Shell is Opalised Shell. The corresponding national-standard name is opal (GB/T 16552—2017 Table A.2), and opalised shell is not listed in the original. Names in four languagesNames are given in four languages: Simplified Chinese, Traditional Chinese, English (Opalised Shell) and Japanese; the Japanese item is the GSJ (Japan) formal name, but Japan's GSJ wording for this name is not yet verified by this site, and the entry's own Japanese name is shown. Trade and mineralogical namesThe mineralogical name is fossil shell replaced by opal. The name register also lists the English synonym Shell Opal (based on CIBJO Gemstone Book 2022 Annex 8), with no restriction recorded on its use as a name. Institution usageOn authority wording, the CIBJO Gemmological Laboratories Book 2024 records Opalised Shell; the FTC text on file has no wording for this name (checked); GIA and Gem-A are unchecked; the English name shown for bodies without a recorded wording comes from the entry itself and does not represent their position.
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) | ||||
| Shell OpalEnglish | synonym | matched automatically | no restriction recorded | CIBJO Gemstone Book 2022 Annex 8 “Shell Opal” |
How each authority words this name: see Names › Authority wording.
- GSJ (Japan) formal name
- オパール化貝殻 (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)
- 欧泊 (GB/T 16552—2017 Table A.2)
- Mineralogical name
- Fossil shell replaced by opal
- Trade names and synonyms
- Shell Opal (synonym)
Four-language names
- 简体中文
- 欧泊化贝壳
- 繁體中文
- 歐泊化貝殼
- English
- Opalised Shell
- 日本語
- オパール化貝殻
See also: Misleading and prohibited trade names · Supplementary name index
Classification
- Origin
- Natural materials
- Material domain
- Natural inorganic gem materials
- Browsing family
- Opal and natural amorphous silica materials
- Identity tier
- Mineral / gemstone variety
- Mineral chemical class
- Not applicable
- Construction or treatment status
- Natural identity; treatment status unknown
What it is and how it's identified
The composition (phase) of Opalised Shell is opal. Registered from the clause for opal (the whole class): overall structure amorphous; cleavage and birefringence both none; when untreated, pleochroism none; optical character isotropic, with anomalous extinction often seen in fire opal; play-of-colour can occur, and the cat's-eye effect can occur but is rare; when untreated, lustre is vitreous to resinous. Density 1.25–2.23 g/cm³, derived from the clause value 2.15 and its permitted tolerance of +0.08/−0.90, registered for opal (the whole class); refractive index 1.37–1.47, derived from the clause value 1.450 and its permitted tolerance of +0.020/−0.080, registered for opal (the whole class), using the overall clause value, with the clause having finer sub-ranges. Mohs hardness 5–6.5; the cited clause covers the whole species, of which Opalised Shell is only one variety. Stability is registered as prone to breaking, porous, liable to crack on losing water and sensitive to sudden temperature changes, treatment status untreated.
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 opal (whole class), so they are not repeated on the Opalised Shell page; see the material facts on the Opal entry.
Items below that name the class they cover give the cited clause’s value for that whole class, not a measurement of Opalised Shell (how to read class-level values).
Definition
Opalised Shell — composition (phase): opal
CIBJO Gemmological Laboratories Book 2024
Source & check 2026-09-22
- Source
- Fact checked on
- 2026-09-22
Structure
Opalised Shell — structure (overall): amorphous (not crystalline)
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Durability
Opalised Shell — Mohs hardness: 5–6.5
GIA Opal
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))
- Qualification
The clause we cite covers the whole species. This entry is only one variety of it.
Source & check 2026-09-20
- Source
- GIA Gem Encyclopedia — Opal (Gemological Institute of America): Section “FACTS”, “Mohs Hardness” (locator checked 2026-09-28)
- Fact checked on
- 2026-09-20
Opalised Shell — cleavage: none
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Opalised Shell — stability: chips and cracks easily; porous, so keep it away from coloured liquids; will craze if it loses water, so keep it away from heat and dry air; avoid sudden temperature changes; no ultrasonic cleaning.
CIBJO Gemstone Book 2022
CIBJO Gemstone Book §6.2:special-care code b, special-care code c, special-care code d, special-care code e, special-care code j
- Treatment state
- untreated
Source & check 2026-09-21
- Source
- Fact checked on
- 2026-09-21
Physical properties
Opalised Shell — density: 1.25–2.23 g/cm³
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
- Qualification
The clause we cite gives 2.15, with a stated tolerance of +0.08 / -0.90. The range shown here is worked out from that.
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Optical
Opalised Shell — optic character: isotropic; fire opal commonly shows anomalous extinction.
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Opalised Shell — refractive index: 1.37–1.47
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
- Qualification
The clause we cite gives 1.450, with a stated tolerance of +0.020 / -0.080. The range shown here is worked out from that.
- General note
- the clause has finer ranges
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Opalised Shell — birefringence: none
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Opalised Shell — pleochroism: none
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Opalised Shell — optical effect (play of colour): seen
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Opalised Shell — optical effect (chatoyancy): seen; rare
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
- CIBJO definitions
- play of colour, chatoyancy
Quality factors
Opalised Shell — quality factor (lustre): vitreous to resinous
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (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): 1.42 / 1.74; Closest values: 1Synthetic Opal 1.45 / 2.08, 2Mammoth Ivory 1.53 / 1.75, 3Ivory 1.535 / 1.85, 4Hackmanite 1.483 / 2.27, 5Chrysocolla 1.516 / 2.2
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
Clause-level definitions
Opalised Shell is defined directly in CIBJO Gemmological Laboratories Book 2024, with the registered term Opalised Shell; the definition essentially describes shell that has been replaced by opal. The cross-check shows it is defined directly by CIBJO, and no identically named English entry was found in this edition of the GB/T annex.
This table scrolls sideways on narrow screens.
| Term | Clause | Definition | Cross-check |
|---|---|---|---|
| Opalised shell | 7.2.1.164 | shell that has been replaced by opal | Defined directly by CIBJO; no identically named English entry was found in this edition of the GB/T annex. |
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.
- RRUFF Project, in partnership with the IMA – International Mineralogical Association. IMA Mineral List.
- Geological Survey of Japan, AIST (Japan). Museum mineral specimens, browse by classification.
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 Opal on this site: 17 entries listed by this site's parent-entry links; synthetic, treated and similar entries carry their recorded state after the name; this page (Opalised Shell) is marked.
- Opal
Sub-entries (16)
This table scrolls sideways on narrow screens.
| Page | Content | Status |
|---|---|---|
| Browsing family | Opal and natural amorphous silica materials | Live |
| Related articles | To be added (placeholder) | Planned |
Who maintains this page
The material facts in the Opalised Shell 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; clause-level definitions also carry a cross-check result. The entry exists so that buyers and sellers can check naming, disclosure and material facts item by item.