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Natural inorganic gem materials · Mineral / gemstone variety · Natural identity; treatment status unknown
Opalised Fossil
Also known asFossil Opal
A general term for fossil material replaced by opal, subdivided by the original fossil, such as Opalised Shell and Opalised Wood. Also known as Fossil Opal. 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 Fossil 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 FossilIdentification points from the research literature
- Hardness, density and refractive index of Opalised FossilEach value with its source and check date
- Opalised Fossil 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 FossilThe 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
An opalised fossil is biological remains (such as shell or wood) replaced by opal (hydrated amorphous silica); the replacement process can preserve the form and structure of the original organism. A related, confirmable mechanism is that opal can form under the structural influence of a pre-existing fibrous mineral, i.e. the pre-existing fibrous mineral is replaced by opal, producing a fibrous internal structure; this supports the notion of biological/mineral remains being replaced by silica. The cited authoritative sources give no dedicated account of the opalised fossil as a separate variety, so its formation statements are largely grade C. Opal itself is hydrated amorphous silica and its genesis is complex.
Scope: Applies to explaining that opalised fossil is a variety in which biological remains are replaced by opal, and cites the documented mechanism of 'replacement of pre-existing fibrous minerals'; it does not constitute a formation conclusion specific to this variety (the cited sources have no specific content).
Sources G&G Spring 2017 · G&G Fall 2019 · G&G Winter 2023
Where it forms
Sedimentary or weathering settings with silica-bearing fluids, where biological remains are replaced by silica.
Sources G&G Spring 2017 · G&G Winter 2023
Natural opal forms in sedimentary-type or volcanic-type environments.
Sources G&G Spring 2017 · G&G Winter 2023
How it forms, step by step
Biological remains (shell, wood, etc.) are buried and become the fossil substrate.
Silica-bearing fluids penetrate along the remains' structure and progressively replace the original mineral.
The replacing medium precipitates as hydrated amorphous silica (opal), potentially preserving the original biological structure.
An opalised fossil forms; because opal is water-bearing, it carries a risk of crazing and cracking.
Link to appearance
An opalised fossil often preserves the external form and internal structural outlines of the original remains while possibly showing opal's body color and play-of-color; its appearance depends on the remains' form and the properties of the replacing opal.
Sources G&G Fall 2019
Timescale
The cited sources give no replacement duration or age for opalised fossils.
Sources G&G Spring 2017 · G&G Fall 2019 · G&G Winter 2023
Related base materials
- Biological remains (fossil substrate): replacement-substrate | Biological remains are replaced by opal (silica), preserving the original biological structure
- Opal (hydrated amorphous silica): replacing-mineral | Opal is the replacing medium, filling/replacing the original biological hard parts
Supporting findings
Opal can form under the structural influence of a pre-existing fibrous mineral, i.e. the pre-existing fibrous mineral is replaced by opal; this shows silica can replace existing minerals and is supporting evidence for opalised fossil formation.
Evidence grade CScope: The observation comes from a chatoyant zone in Ethiopian opal, a single-case mechanism; the source set does not directly address fossil replacement, hence grade C.
Sources G&G Spring 2017
The genesis of precious opal is quite complex; opal forms in sedimentary-type or volcanic-type settings, but the source set provides no fossil-specific setting or age.
Evidence grade CScope: A general framework for opal settings; does not assert a specific setting for opalised fossils.
Sources G&G Winter 2023
Recorded localities
Authoritative sources list the localities below; the list is not exhaustive.
Oceania
- AustraliaPer Geoscience Australia, the only place where opalised animal and plant fossils have been found
Sources Geoscience Australia
- Lightning Ridge, New South Wales, Australia
Sources Geoscience Australia · Gems & Gemology Winter 2015
- Coober Pedy, South Australia, AustraliaFind site of the best-known opalised fossil
Sources Geoscience Australia
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
In an opalised fossil the opal-replaced portion still follows opal's optics: regularly stacked submicroscopic silica spheres diffract white light into spectral colors that change with viewing angle (sphere size and spacing set the color range). This mechanism acts on the array of silica spheres produced by replacement and is independent of the fossil's original biological form.
Evidence grade AScope: Applies to material in which opal has replaced biological remains; the mechanistic account does not prove any specimen's locality, naturalness, or treatment.
Sources GIA Opal Description · G&G Summer 2025
Separately, opal can replace a pre-existing fibrous mineral while following its form, thereby preserving a fibrous internal structure; this inherited form and texture is a different aspect from the sphere-diffraction structure that produces the play-of-color—the former accounts for the preservation of the remains' outline, the latter for the play-of-color itself.
Evidence grade BScope: A supporting point on how the original biological/mineral structure is preserved; the sources contain no optical study specific to opalised fossils, so the mechanism is by extension.
Sources G&G Spring 2017 · GIA Opal Description
How it is identified
Materials with a play-of-colour appearance such as opalised fossils cannot be judged by eye as natural opal, synthetic or imitation: some products sold for their play-of-colour are in fact opal simulants whose composition (for example the absence of water) does not match natural opal. Identification requires laboratory methods that compare gemmological properties and separate the material from natural opal; the play-of-colour appearance is not by itself diagnostic. Limitations: a play-of-colour look can be reproduced by several materials, and the conclusion must be issued by an accredited gemological laboratory through testing rather than inferred from appearance.
Evidence grade AScope: Identification and separation points; testing must be performed by an accredited gemological laboratory, and no self-test thresholds are given.
Other research findings
An opalised fossil is biological remains (fossil material) replaced by opal, an opal fossil variety rather than an independent mineral species.
Evidence grade CScope: An identity-boundary statement; the source set does not specifically document opalised fossils, so grade-C inference from the definition.
Sources G&G Fall 2019
The material doing the replacing is opal, a hydrated amorphous silica; where the water content is high, it tends to craze and crack when allowed to dry out.
Evidence grade AScope: A general property of opal material applicable to the replacing medium in an opalised fossil; not specific to the variety.
Sources G&G Fall 2019
Limits of this research
- Research status: Formation, occurrence or other material-specific findings found in authoritative sources
- No usable source specific to opalised fossils/shell/wood has been found, so formation statements are largely grade C.
- The replacement mechanism is supported only by the 'fibrous mineral replaced by opal' observation; fossil-specific source support is lacking.
- Replacement duration, age, and a fossil-specific setting lack source support.
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 fossil (state the fossil)
CIBJO Gemmological Laboratories Book 2024 §7.2.1.163 “Opalised fossil” · IMA Mineral List 2026-07 “Opal”
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
Opalised Fossil's material identity has open questions that are not fully settled; this site lists it as "Opalised fossil (state the fossil)" pending further verification.
Evidence
- CIBJO · 7.2.1.163
What this means for you
Before buying, check the recommended listing name "Opalised fossil (state the fossil)" 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
- 2026-09-26
Care and wear
No care facts are recorded for this entry itself. Its parent material has care records: see Opal.
VerifyNames, standards, technical facts, clauses and sources
Names
Besides Opalised Fossil, the name register records "Fossil Opal", for which no restriction on use as a name is recorded.
Institution usageIn the names comparison for Opalised Fossil, the CIBJO Gemmological Laboratories Book 2024 lists Opalised Fossil; the FTC source text has no wording for this name (checked); GIA and Gem-A have not been checked, and bodies without a recorded wording show the entry's own English name, which does not represent their position. Chinese standard nameThis name itself is not listed in GB/T 16552—2017, and the GB/T naming counterpart is Opal (Table A.2). Trade and mineralogical namesThe trade name or synonym Fossil Opal is registered as a market name with no restriction on its use as a name, based on this site's ruling (no supporting clause). Recommended listing nameThe suggested listing name is Opalised Fossil (stating the type of fossil). Names in four languagesThe names are given in Simplified Chinese, Traditional Chinese, English (Opalised Fossil) and Japanese (pending check); the Japanese GSJ formal name has not yet been checked by this site, and the entry's own Japanese name (pending check) is shown. Trade and mineralogical namesThe mineralogical name is Opalised fossil, meaning fossil material replaced by opal.
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) | ||||
| Fossil OpalEnglish | market name | matched automatically | no restriction recorded | editorial ruling (no clause cited) |
How each authority words this name: see Names › Authority wording.
- GSJ (Japan) formal name
- オパール化石（待核） (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
- Opalised fossil: fossil material replaced by opal
- Trade names and synonyms
- Fossil Opal (market name)
Four-language names
- 简体中文
- 欧泊化化石
- 繁體中文
- 歐泊化化石
- English
- Opalised Fossil
- 日本語
- オパール化石（待核）
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 Fossil is opal. Taking the values for opal (the whole class) from GB/T 16553—2017, density is 1.25–2.23 g/cm³, the standard giving 2.15 with a stated tolerance (+0.08/−0.90), from which the limits are derived; refractive index is 1.37–1.47, the standard giving 1.450 with a stated tolerance (+0.020/−0.080), from which the limits are derived; the overall refractive index value is registered, as the clause has further sub-grades; there is no birefringence, taken for opal (the whole class) from GB/T 16553—2017; and Mohs hardness is 5–6.5, taken for opal (the whole class). Whole-class features such as chemical composition, fluorescence, UV-visible spectrum, infrared spectrum and magnification are registered for opal (the whole class) and are not repeated for Opalised Fossil; see the material facts of the Opal entry. Values taken for opal (the whole class) are not measurements of Opalised Fossil.
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 Fossil 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 Fossil (how to read class-level values).
Definition
Opalised Fossil — composition (phase): opal
CIBJO Gemmological Laboratories Book 2024
Source & check 2026-09-26
- Source
- Fact checked on
- 2026-09-26
Durability
Opalised Fossil — 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))
- What this covers
- opal (whole class)
Source & check 2026-09-26
- Source
- GIA Gem Encyclopedia — Opal (Gemological Institute of America): Section “FACTS”, “Mohs Hardness” (locator checked 2026-09-28)
- Fact checked on
- 2026-09-26
Physical properties
Opalised Fossil — 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 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Optical
Opalised Fossil — 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 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Opalised Fossil — birefringence: 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
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
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.
- eCFR (United States). 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 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 Fossil) 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 Fossil 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.