On this page
UnderstandWhat it is, what people ask, how far this page goes
Natural inorganic gem materials · Mineral / gemstone variety · Natural identity; treatment status unknown
Precious Opal
A variety of natural opal showing play-of-colour; opalescence must be kept distinct from play-of-colour. 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 Precious Opal 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 Precious OpalIdentification points from the research literature; how far lab testing can go
- Treatments and enhancements of Precious OpalThe enhancements and treatments the GB/T and JJA/AGL tables list
- Caring for and wearing Precious OpalCleaning, heat, chemicals and light, as the cited sources give them
- Hardness, density and refractive index of Precious OpalEach value with its source and check date
- Precious Opal 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 Precious OpalThe listing name to use; what must be disclosed before sale
- Standard clauses that define Precious OpalWhich 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
Precious opal is natural opal that displays play-of-color, as opposed to common opal, which does not. It is hydrated amorphous silica: rainwater dissolves silica, silica gel deposits in cracks in underground rock, and it consolidates as it loses water. Its play-of-color arises from ordered stacks of monodisperse silica spheres—spheres (or opal-CT lepispheres) roughly 250 nm in size—that form a 3D photonic crystal and diffract visible light so that spectral colors change with viewing angle. The diffracted color depends on sphere size: spheres about 0.1 µm across give violet, and spheres about 0.2 µm give red.
Scope: Applies to the genesis and optical mechanism of natural precious opal showing play-of-colour; does not apply to common opal or to man-made/imitation opal.
Sources GIA Opal Description · G&G Summer 2025 · G&G Fall 2013 +1
Where it forms
A natural setting where silica-bearing groundwater deposits and consolidates in rock cracks
Sources G&G Fall 2013
How it forms, step by step
Rainwater dissolves silica, forming a silica gel that deposits in cracks in underground rock.
Sources G&G Fall 2013
Silica spheres stack into an ordered, monodisperse array (a 3D photonic crystal) and consolidate into solid opal as the gel dehydrates.
Visible light diffracts between the ordered spheres; the diffracted color changes with sphere size and viewing angle, producing play-of-color.
Sources G&G Summer 2025 · G&G Fall 2013 · GIA Opal Description
Link to appearance
Play-of-color is produced by diffraction from the internal 3D periodic structure of ordered silica spheres; spheres of about 250 nm diffract visible light, and different sizes give different colors.
Sources G&G Summer 2025 · G&G Fall 2013 · GIA Opal Description
Timescale
The cited sources give no absolute timescale for precious opal formation.
Sources G&G Fall 2013
Supporting findings
Opal is a hydrated silica that is amorphous or only poorly crystallized, having solidified from a silica gel.
Evidence grade AScope: A general formation statement for opal, including precious opal.
Sources G&G Fall 2013
Recorded localities
Authoritative sources list the localities below; the list is not exhaustive.
Africa
- Wegel Tena, Wollo, Ethiopia
Sources G&G Fall 2013
Asia
- Java, Indonesia
Sources G&G Fall 2013
North America
- Mexico
Sources G&G Fall 2013
- United States
Sources G&G Fall 2013
- Honduras
Sources G&G Fall 2013
South America
- Brazil
Sources G&G Fall 2013
Oceania
- Australia
Sources G&G Fall 2013
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.
Causes of colour and optical effects
Play-of-color can appear only in opal-AG and opal-CT that are microscopically ordered. In these, stacked silica spheres or lepispheres roughly 250 nm across act as three-dimensional photonic crystals and diffract visible light, which is what generates the effect.
Evidence grade AScope: The mechanism applies to natural precious opal; the sphere size (~250 nm) is a typical value.
The diffracted color depends on silica-sphere size: spheres about 0.1 µm give violet, about 0.2 µm give red, and intermediate sizes give the remaining spectral colors.
Evidence grade AScope: A qualitative-to-semi-quantitative relationship; the source gives no full diffraction model.
Sources GIA Opal Description
How it is identified
Strong play-of-colour alone does not prove that a stone is natural opal. Laboratory-made products with play-of-colour exist, and some contain no water, so they do not match natural opal's composition and are classed as imitations rather than synthetic opal. Separating natural opal from such laboratory material, or from assembled or composite material, relies on measuring gemological properties and composition; because the display can look very similar, the identification should be made by a qualified gem laboratory.
Evidence grade AScope: Applies where natural, synthetic and imitation opal must be distinguished; no self-administered criteria are given.
Other research findings
Precious opal is natural opal that displays play-of-color, which is how it is separated from common opal, which does not.
Evidence grade AScope: The gemological split between precious and common opal; it is not a formation definition.
Sources GIA Opal Description · G&G Summer 2025
Opal can contain up to about 20% water trapped in its silica structure.
Evidence grade BScope: A general figure; it does not separate precious and common opal.
Sources GIA Opal
Limits of this research
- Research status: Formation, occurrence or other material-specific findings found in authoritative sources
- The cited sources give no absolute timescale for precious opal formation and do not cover all locality differences.
- Quantitative descriptions of play-of-color remain incomplete in the literature.
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
- Precious Opal (showing play-of-colour)
IMA Mineral List 2026-07 “Opal” · CIBJO Gemmological Laboratories Book 2024 §7.2.1.162 “Opal”
Construction or treatment status
Natural identity; treatment status unknown
Treatments recorded for this material: Filled / Fracture-filled (via parent material), Dyed (via parent material), Coated (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
Precious Opal is Opal displaying play-of-colour; use "Precious Opal (showing play-of-colour)" for listings, and disclose any known treatments.
Evidence
- CIBJO · 7.2.1.162
What this means for you
Before buying, check the recommended listing name "Precious Opal (showing play-of-colour)" 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.
The item-by-item care advice that the GIA Care and Cleaning Guide gives for the whole class, and so for Precious Opal, is set out in full under care on the Opal 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 |
|---|---|---|---|---|---|
| Filling | — | — | Graded — see §5.4.1, Table 1 | — | — |
| Dyeing | — | — | Treatment | — | — |
| Coating | — | — | Treatment | — | — |
“—” means the cited source gives no value for that item, not “none”. Source: GB/T 16552—2017 Table B.1 (row for Opal)
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
Chinese standard nameThe GB/T naming counterpart for Precious Opal is Opal (GB/T 16552—2017 Table A.2), and this name itself is not listed in the standard; the mineralogical name is Opal displaying play-of-colour. Names in four languagesThe four-language names are given in Simplified Chinese, Traditional Chinese, English (Precious Opal) and Japanese (a transliteration). Recommended listing nameThe suggested listing name is Precious Opal (showing play of colour). Institution usageAmong authority wordings, Precious Opal is the entry's own English name; the FTC and CIBJO source texts have no wording for Precious Opal (checked), and GIA and Gem-A have not been checked; bodies without a recorded wording show the entry's own English name, which does not represent their position. Japanese namesOn the Japanese GSJ side, the entry's own Japanese name is a transliteration, and the Japanese GSJ wording for it has not yet been checked.
- GSJ (Japan) formal name
- プレシャスオパール (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
- Opal displaying play-of-colour
Four-language names
- 简体中文
- 贵欧泊
- 繁體中文
- 貴歐泊
- English
- Precious Opal
- 日本語
- プレシャスオパール
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
Treatments listed by standards
For Precious Opal, this site applies the Opal row of GB/T 16552—2017 Annex B, Table B.1: filling is classed by graded definitions, dyeing is listed as treatment and coating is listed as treatment. The notes to Table B.1 and the graded definitions are on the Gem treatments page.
GB/T 16552-2017, Annex B, Table B.1
Under the “Opal” row of Table B.1: Filling Graded — see §5.4.1, Table 1; Dyeing Treatment; Coating Treatment. The full row, with the effect column, is on the Opal 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
Among the material facts for Precious Opal, structure, cleavage, density, optic character, refractive index, optical effects and lustre are written by the cited clauses for opal (the whole class), not measured on Precious Opal separately. The structure (overall) is amorphous; there is no cleavage; density is 1.25–2.23 g/cm³, derived from the clause value of 2.15 with a tolerance of +0.08/−0.90; the optic character is isotropic, with fire opal often showing anomalous extinction; refractive index is 1.37–1.47, derived from the clause value of 1.450 with a tolerance of +0.020/−0.080, the clause having further sub-grades, with the overall value used; as optical effects, play-of-colour can be seen, and the cat's-eye effect can be seen but is rare; the quality factor (lustre) is vitreous to resinous, untreated. Mohs hardness is 5–6.5; the cited clause covers the whole species opal, Precious Opal being a variety within opal. Chemical composition, fluorescence, UV-visible spectrum, infrared spectrum and magnification are given for opal (the whole class); see the Opal entry.
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 Precious Opal 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 Precious Opal (how to read class-level values).
Structure
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
- Qualification
The row we cite covers opal as a whole, and this entry is one of its varieties. The row states a property that holds for every piece alike — whether it dissolves, whether it cracks. That kind of property carries over to this entry. A numeric range such as hardness would not.
Source & check 2026-09-21
- Source
- Fact checked on
- 2026-09-21
Physical properties
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
Precious Opal — 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
Testing and market
The CIBJO laboratory protocol covers Precious Opal under “Opal” (§6.1.56): see identification on the Opal entry; the required tests are listed on the CIBJO laboratory protocol table for Opal.
How far testing can go
Each row's “Source & check” expands to the full source; the date beside it is the fact check date.
Clause-level definitions
Precious Opal is registered in the clause-level definitions under the entry Precious Opal, with the corresponding clause in GB/T 16552-2017 Annex A, and the definition given is Opal displaying play-of-colour. The cross-check result is that GB/T lists it directly, and no CIBJO entry under the same standalone heading was found.
This table scrolls sideways on narrow screens.
| Term | Clause | Definition | Cross-check |
|---|---|---|---|
| Precious Opal | GB/T 16552-2017 Annex A | Opal displaying play-of-colour | Listed directly by GB/T; CIBJO carries no standalone entry under the same name. |
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 (Precious Opal) 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 |
| Treatments recorded for this material | Filled / Fracture-filled (via parent material), Dyed (via parent material), Coated (via parent material) | Live |
| Related articles | To be added (placeholder) | Planned |
Who maintains this page
The material facts in the Precious Opal 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.