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Natural inorganic gem materials · Natural amorphous gem material / mineraloid · Natural identity; treatment status unknown

Opal

Also known asOolitic opal

A hydrous amorphous silica gem material whose basic name in GB/T 16552-2017 Table A.2 is "Opal". Natural, synthetic, assembled and imitation opal must be distinguished. 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.

Edited by JuneReviewed by RalphLast updated 2026-10-12Who maintains this page

FAQCare and wear

Questions people ask about Opal

Every sentence below is followed by the registered fact it rests on; answers add no new facts and promise no outcome.

Q1How is natural opal told apart from synthetic or imitation opal?

Not by eye: natural, synthetic and imitation opal can look very much alike, and the unaided eye cannot reliably separate them.G&G Fall 1987A laboratory has to test composition and internal structure, such as whether water is present and what structural features occur, using microscopy and instruments.G&G Fall 1987Synthetic opal is commercially available.CIBJO Gemstone Book 2022Assembled opals also exist: opal too thin to be sturdy is backed with black onyx, plastic or natural matrix as a doublet, and triplets add a clear cap of rock crystal, plastic, glass or synthetic corundum.GIA Introduction to Imitation GemsWhether one particular bead is natural, treated or synthetic — only a laboratory report can settle that.What this page can answer

Q2What causes the flashes of colour in opal?

Diffraction: play-of-colour in precious opal comes from visible light diffracting off silica spheres of one size arranged in an orderly way.G&G Fall 2013The colour seen depends on the sphere size: roughly 0.1 micron gives violet and about 0.2 microns gives red.G&G Fall 2013Play-of-colour opal with a white body colour may be called white opal, one with a dark body colour black opal, and orange to red opal fire opal.GB/T 16553—2017

Q3Can opal get wet, and how should you clean it?

Warm soapy water is the only safe way to clean opal.GIA Gem Encyclopedia (care guide)It is porous, so it should be kept away from coloured liquids, and it must not be cleaned ultrasonically.CIBJO Gemstone Book 2022Steam cleaning should be avoided as well.Gems & Gemology 1987 (Martin)Hydrofluoric acid and caustic alkaline solutions attack it.GIA Gem Encyclopedia (care guide)Some opal, called hydrophane, is porous enough to absorb liquids so that its look and even its weight change; it can come from any locality, but Ethiopian deposits yield an unusually high proportion, and in one laboratory test immersed slices gained weight and became more transparent.LMHC Information Sheet #13 · G&G Spring 2016

Q4Is opal durable enough for an everyday ring?

Only with care: opal's hardness varies from Mohs 5 to 6.5 and its toughness from very poor to fair, so it is fine for jewellery but should be worn carefully to avoid scratches and breaks.GIA Gem Encyclopedia (care guide)It chips and cracks easily.CIBJO Gemstone Book 2022Prong and bezel settings call for caution, and flush, channel and pavé are best avoided.Gems & Gemology 1987 (Martin)

Q5Why can opal crack, cloud or lose its colour?

Opal will craze if it loses water, so it should be kept away from heat and dry air and from sudden temperature changes.CIBJO Gemstone Book 2022Heat from intense light can cause a network of fine cracks called crazing.GIA Gem Encyclopedia (care guide)For display, case heat and sunlight are best avoided and artificial light calls for caution, because heat and strong light speed up dehydration.Gems & Gemology 1987 (Martin)Heat can turn opal white or brown and destroy its play-of-colour.Gems & Gemology 1987 (Martin)

Q6How does opal form?

Opal is hydrated silica that is amorphous or only poorly crystallized, produced when a silica gel solidifies.G&G Fall 2013In places such as Australia's outback, seasonal rain seeps into underground rock and carries dissolved silica downward.GIA Opal DescriptionDuring dry periods much of the water evaporates, leaving solid silica in cracks and between layers of sedimentary rock, forming opal.GIA Opal DescriptionIn Ethiopia, opal forms in cavities of weathered volcanic ignimbrite, where silica gel partly fills the cavity.G&G Fall 2013The sources give no numeric age or absolute timescale for opal formation.GIA Opal Description · G&G Fall 2013

Q7Is opal treated, filled or dyed?

Yes: some opal is filled with injected colourless plastic, which gives a low density and shows in infrared spectra.GB/T 16553—2017In dyed opal, magnification shows the colour unevenly distributed, mostly as fine particles in fissures, between grains or in surface hollows.GB/T 16553—2017For dyed, filled or impregnated opal, CIBJO notes that such treatments are not permanent and that the stone should be kept away from all solvents, including dish-washing liquids, as well as chemicals and heat.CIBJO Gemstone Book 2022Whether one particular bead is natural, treated or synthetic — only a laboratory report can settle that.What this page can answer

Q8What makes a good-quality opal?

According to GIA, opal quality has three main aspects: colour (background colour and play-of-colour), pattern (the arrangement of the play-of-colour) and clarity (transparency and inclusions).GIA Gem EncyclopediaWhatever its colours, play-of-colour must be vivid to rate highly.GIA Gem EncyclopediaTraditionally red is considered the best prominent play-of-colour, orange the next, followed by green; for pattern, connoisseurs prefer large, closely arranged patches of colour to tiny, scattered dots.GIA Gem Encyclopedia

Q9What does opal symbolize, and does it have healing powers?

There is no scientific evidence that opal has healing powers: the UK advertising regulators ASA and CAP state that they have not seen evidence that crystal therapy can treat medical conditions.ASA/CAP guidance (UK)Its recorded cultural role is as the October birthstone: Jewelers of America and Japan's JJA list it for October, and the U.K. National Association of Jewellers lists it as traditional for October.Cultural records

What this page can answer

This page covers

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

Opal is hydrated silica that is amorphous or only poorly crystallized, produced when a silica gel solidifies. In areas such as Australia's outback, rain that falls seasonally penetrates underground rock, transporting dissolved silica downward; when dry spells arrive much of that water evaporates, and solid silica remains in cracks and between the layers of sedimentary rock underground. Sub-microscopic silica spheres arranged on a regular grid constitute precious opal, and their arrangement yields play-of-color. Certain Ethiopian opals, through deposition, polygonization, remobilization and interstitial filling, also develop vertical columnar 'digit' structures.

Scope: Applies to the formation of natural opal in sedimentary and volcaniclastic host-rock environments; the specific observational data for the columnar (digit) structure model come mainly from Ethiopian opal and should not be applied directly to other localities.

Sources GIA Opal Description · G&G Fall 2013

Where it forms

Australia's semi-desert outback: seasonal rains and ancient underground sedimentary rock, with silica precipitating in cracks and between layers.

Sources GIA Opal Description

Cavities in weathered ignimbritic volcanic host rock in Ethiopia, where silica gel partially fills the cavity and develops vertical columnar structures.

Sources G&G Fall 2013

How it forms, step by step

  1. Rain falls seasonally and seeps into underground rock; the dissolved silica it carries—silicon and oxygen combined—moves downward.

    Sources GIA Opal Description

  2. During dry periods much of the water evaporates, leaving solid silica deposits in cracks and between layers of sedimentary rock, forming opal.

    Sources GIA Opal Description

  3. The silica gel solidifies through dehydration and cementation into amorphous hydrated silica, i.e. solid opal.

    Sources G&G Fall 2013

  4. In some Ethiopian opals, silica spheres sediment into vertical columnar structures that later polygonize into juxtaposed sub-grains.

    Sources G&G Fall 2013

  5. A later silica-poor fluid remobilizes the columns by erosion along grain boundaries, rounding them, after which new silica deposits between them and solidifies.

    Sources G&G Fall 2013

Link to appearance

In precious opal, play-of-color results from visible light diffracting off monosized, well-ordered silica spheres; the hue observed depends on sphere size—roughly 0.1 micron gives violet, while about 0.2 microns gives red.

Sources G&G Fall 2013

Timescale

The cited sources describe opal formation through alternating seasonal rains and dry periods and precipitation between sedimentary layers, but give no numeric age or absolute timescale for opal formation.

Sources GIA Opal Description · G&G Fall 2013

Related base materials

  • Silica-bearing fluid (silica gel): Opal forms by the consolidation of hydrous silica gel; silica gel is its direct precursor material

Supporting findings

Opal forms when silica dissolved by seasonal rains precipitates in cracks and between layers of underground sedimentary rock.

Evidence grade A

Scope: Describes opal formation in the Australian sedimentary setting; other genetic types are not covered by this source.

Sources GIA Opal Description

In Ethiopian opals, digit structures develop in partially filled cavities, the columns being vertical with rounded ends pointing upward.

Evidence grade A

Scope: The observations are mainly for Ethiopian opal; the source notes such structures are rare in other deposits.

Sources G&G Fall 2013

Digit structures form in sequence through sphere deposition into columns, polygonization, remobilization and rounding, interstitial filling, and dehydration solidification.

Evidence grade A

Scope: This stepwise model addresses digits in play-of-color opal; the source notes some samples preserve only part of the sequence.

Sources G&G Fall 2013

Recorded localities

Authoritative sources list the localities below; the list is not exhaustive.

Locality sketch mapLand outlines only; the map does not depict any national or regional boundary. Dots mark approximate positions. Base map: Natural Earth (public domain).

Africa

Asia

Europe

North America

South America

Oceania

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

Diffraction of visible light by monosized, well-ordered silica spheres is what generates play-of-color in precious opal.

Evidence grade A

Scope: Applies to regularly packed silica spheres; common opal with irregular packing shows no play-of-color.

Sources G&G Fall 2013

How it is identified

Natural, synthetic and imitation opal can look very much alike, and the unaided eye cannot reliably separate them. A laboratory must test composition and internal structure—for example whether water is present and what structural features occur—using microscopy and instrumental analysis; the finding must come from an accredited body.

Evidence grade A

Scope: Separating natural, synthetic and imitation opal is done in a laboratory and decided by an accredited body.

Sources G&G Fall 1987

Opal doublets and triplets are assembled stones. Natural opal sometimes occurs in layers too thin to be sturdy enough for jewellery, so it is reinforced: black onyx, plastic or natural matrix have served as the bottom layer of opal doublet or triplet cabochons, and opal triplets are capped with a transparent dome of rock crystal, plastic, glass or synthetic corundum.

Evidence grade A

Scope: Describes identification features reported by a laboratory or educational source; it is not a test that a reader can rely on, and any single piece can only be identified by a gemmological laboratory.

Sources GIA Introduction to Imitation Gems

Other research findings

Opal is hydrated silica that is amorphous or only poorly crystallized, produced when a silica gel solidifies.

Evidence grade A

Scope: This compositional definition rests on a review statement about opal genesis within a peer-reviewed article.

Sources G&G Fall 2013

Opal quality has three main aspects: colour (background colour and play-of-colour), pattern (the arrangement of the play-of-colour) and clarity (transparency and inclusions). Whatever its colours, play-of-colour must be vivid to rate highly.

Evidence grade B

Scope: Quality factors as described by one educational source; not a grading standard and not a statement about the value of any piece.

Sources GIA Gem Encyclopedia

Traditionally red is considered the best prominent play-of-colour, orange the next, followed by green; for pattern, connoisseurs prefer large, closely arranged patches of colour to tiny, scattered dots.

Evidence grade B

Scope: Quality factors as described by one educational source; not a grading standard and not a statement about the value of any piece.

Sources GIA Gem Encyclopedia

Hydrophane opal is opal porous enough to absorb liquids to the point that its appearance, and potentially its weight, changes. Such opal can come from any locality, but deposits in Ethiopia yield an unusually high proportion of it. In a laboratory test on hydrophane opal slices, immersion in water caused obvious weight gain and made the slices more transparent.

Evidence grade A

Scope: Explains why some opal changes in water, as described by a laboratory harmonisation body and one laboratory note; it is not a test to perform at home (the laboratory procedure notes that immersion can crack the stone) and does not say whether any particular opal is hydrophane.

Sources LMHC Information Sheet #13 · G&G Spring 2016

Limits of this research

  • Research status: Formation, occurrence or other material-specific findings found in authoritative sources
  • The cited sources give no numeric age or absolute timescale for opal formation.
  • The digit-formation model addresses Ethiopian play-of-color opal; the source states these structures are rare in other deposits.

Methodology

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
Opal

GB/T 16552—2017 Table A.2 “Opal” · IMA Mineral List 2026-07 “Opal”

Construction or treatment status

Natural identity; treatment status unknown

Treatments recorded for this material: Coated, Filled / Fracture-filled, Dyed, Impregnated

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

Opal is often subjected to coating. Under GB/T 16552-2017, coating counts as a treatment and must be stated in the name (§5.4.2.2 a). Do not assume it is unenhanced.

Evidence

  • GB/T · Table A.2, row 欧泊 (Opal)

What this means for you

Before buying, check the recommended listing name "Opal" 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

care (cleaning)2 sources compared

Opal — care (cleaning): Warm soapy water is the only safe way to clean it

GIA Gem Encyclopedia (care guide)

Treatment state
any treatment state
Source & check 2026-09-26
Source
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2026-09-26

Opal — care (cleaning): ultrasonic and steamer: avoid; note: avoid thermal shock.

Gems & Gemology 1987 (Martin)

Treatment state
untreated
Source & check 2026-09-26
Source
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2026-09-26

Opal — care (light): Heat from intense light can cause a network of fine cracks (crazing)

GIA Gem Encyclopedia (care guide)

Treatment state
any treatment state
Source & check 2026-09-26
Source
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2026-09-26

care (chemicals)2 sources compared

Opal — care (chemicals): Hydrofluoric acid and caustic alkaline solutions attack it

GIA Gem Encyclopedia (care guide)

Treatment state
any treatment state
Source & check 2026-09-26
Source
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2026-09-26

Opal — care (chemicals): Treatments with dye, oil, resin, wax or plastic are not permanent; keep it away from all solvents (including dish-washing liquids), chemicals and heat.

CIBJO Gemstone Book 2022

CIBJO Gemstone Book §6.2:special-care code k

Treatment state
treated
Only after these treatments
dyeing, filling, impregnation
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

Opal — care (wear): Hardness varies with composition and formation (Mohs 5 to 6.5) and toughness ranges from very poor to fair: fine for jewellery, but wear it carefully to avoid scratches and breaks.

GIA Gem Encyclopedia (care guide)

Treatment state
any treatment state
Source & check 2026-09-26
Source
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2026-09-26

Opal — care (special care): cat's-eye opal impregnated with colourless resin to improve durability and appearance: special care (SC); avoid heat and ultrasonic cleaning.

AGTA Gemstone Information Manual, 17th ed.

Treatment state
treated
Only after these treatments
impregnation
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

Opal — care (setting): prong and bezel: use caution; flush, channel and pavé: avoid.

Gems & Gemology 1987 (Martin)

Treatment state
untreated
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

Opal — care (repair): polishing: use caution; jeweller's torch, pickle pot and thermal shock: avoid; note: heat turns it white or brown and kills the play-of-colour; thermal shock causes cracking, crazing and fracture.

Gems & Gemology 1987 (Martin)

Treatment state
untreated
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

Opal — care (display): artificial light: use caution; heat in the display case and sunlight: avoid; note: heat and strong light speed up dehydration, leading to crazing or fracture.

Gems & Gemology 1987 (Martin)

Treatment state
untreated
Source & check 2026-09-26
Source
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.

Common care questions and matching entries

CleaningCare of Opal: GIA states that, in any treatment state, the only safe cleaning method is warm soapy water; on chemicals, in any treatment state, Opal is attacked by hydrofluoric acid and caustic alkaline solutions; on heat and light, in any treatment state, heat from strong light can cause a network of fine cracks (crazing) in Opal; on wear, in any treatment state, hardness varies with composition and formation conditions (Mohs 5–6.5) and toughness is very poor to fair, so it is suitable for jewellery but must be worn with care to avoid scratching or breaking. Martin, Gem DurabilityMartin's Gemstone Durability: Design to Display states that, on cleaning, ultrasonic cleaning, steam cleaning and thermal shock should be avoided for untreated Opal; on repair, the torch and post-soldering pickling solution should be avoided for untreated Opal, as heat can turn it white or brown and destroy its play-of-colour; on display, for untreated Opal, artificial light calls for caution and display-case heat and daylight should be avoided. CIBJOThe CIBJO Blue Book (Official Gemstone Book, Coloured Stone Commission 2022) states that, on chemicals, treated Opal, and specifically dyed, filled or impregnated Opal, should be kept away from all solvents (including all kinds of detergent), chemicals and high temperatures.

This table scrolls sideways on narrow screens.

QuestionMatching entries
Can it go in an ultrasonic or steam cleaner?Warm soapy water is the only safe way to clean it; ultrasonic and steamer: avoid; note: avoid thermal shock
Heat and thermal shockpolishing: use caution; jeweller's torch, pickle pot and thermal shock: avoid; note: heat turns it white or brown and kills the play-of-colour; thermal shock causes cracking, crazing and fracture
Light and displayHeat from intense light can cause a network of fine cracks (crazing); artificial light: use caution; heat in the display case and sunlight: avoid; note: heat and strong light speed up dehydration, leading to crazing or fracture
Contact with acids, alkalis and household chemicalsHydrofluoric acid and caustic alkaline solutions attack it; Treatments with dye, oil, resin, wax or plastic are not permanent; keep it away from all solvents (including dish-washing liquids), chemicals and heat
Wearing limits (knocks, settings)Hardness varies with composition and formation (Mohs 5 to 6.5) and toughness ranges from very poor to fair: fine for jewellery, but wear it carefully to avoid scratches and breaks; prong and bezel: use caution; flush, channel and pavé: avoid

Care entries by treatment state

The same entries grouped by the treatment state each applies to; each links to its full wording and source. Letters are CIBJO special-care codes, explained on the gemstone care page.

any treatment state

untreated

treated (Only after these treatments: dyeing, filling, impregnation)

treated (Only after these treatments: impregnation)

Durability and wearing

Toughness

Toughness very poor to fair

Sources GIA Gem Encyclopedia (care guide)

Treatments and identification leads

Registered treatmentsThe treatments recorded for Opal are irradiation, filling, coating and dyeing. Values not givenIrradiation has the status treated, stability not stable and prevalence rare, with identification leads and source both marked “—”. Filling and coating have the status treated, and dyeing the status colour-treated; the stability and prevalence of filling, coating and dyeing are all marked “—”. The lead for filling is impregnation with colourless plastic and a low density, with characteristic filler absorption bands in the infrared spectrum; for coating, abnormal surface lustre and local peeling of the coating under a loupe, possible anomalous refractive index readings, and characteristic peaks of the coating in infrared and Raman spectra; for dyeing, uneven colour distribution, often as tiny particles gathered in fractures, grain boundaries or surface pits. A record of “—” means the source gives no value, not “none”.

This table scrolls sideways on narrow screens.

TreatmentMarket frequency (CIBJO)Stability (CIBJO)GB/T 16552 categoryJJA/AGL listingIdentification leads
irradiationStatus: treatedrarelyunstable———
fillingStatus: treated——Graded — see §5.4.1, Table 1—See identification leads
coatingStatus: treated——Treatment—See identification leads
dyeingStatus: colour treated——Treatment—See identification leads
impregnation with a colourless agent———impregnation with a colourless agent—

Where each treatment acts (schematics): Irradiated · Filled / Fracture-filled · Coated · Dyed

“—” means the cited source gives no value for that item, not “none”. Source: CIBJO Gemstone Book 2022 · GB/T 16553—2017; GB/T 16552—2017 Table B.1; JJA/AGL 2025

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

How lab reports word it

Report wording for Opal is set out under report wording on the Hydrophane Opal entry.

VerifyNames, standards, technical facts, clauses and sources

Names

Besides Opal, the name register records "Oolitic opal", for which no restriction on use as a name is recorded (CIBJO Gemmological Laboratories Book 2024 §7.2.1.161 “Oolitic opal”).

Standard and listing namesThe Chinese formal name for Opal comes from GB/T 16552—2017 Table A.2, and the suggested listing name is Opal. Names in four languagesNames are given in Simplified Chinese, Traditional Chinese, English (Opal) and Japanese; the GSJ (Japan) formal name is the Japanese term tanpakuseki (opal). Trade and mineralogical namesThe mineralogical name is natural opal, hydrous amorphous silica. Institution usageOn authority wording, GIA, FTC and CIBJO list Opal; Gem-A is unchecked, and bodies without a recorded wording show the entry's own English name, which does not represent their position. Qualifiers stripped from the headline names: FTC's concern disclosure of treatments and the naming of synthetics and imitations, and CIBJO's require disclosure when treated, synthetic or assembled. National standard namesNational and regional formal names: Germany (RAL) and the European Union both use Opal; France (DGCCRF) uses Opale, noting that origin and treatment must be disclosed; Australia (GAA) uses Opal, and the Australian Opal Code requires disclosure of type, origin and treatment.

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.

NameTypeMatchingUse as a nameBasis
no restriction recorded (1)
Oolitic opalEnglishnarrower name (variety or subtype)matched automaticallyno restriction recordedCIBJO Gemmological Laboratories Book 2024 §7.2.1.161 “Oolitic opal”

How each authority words this name: see Names › Authority wording.

Authority wording

GIA, FTC, CIBJO, Gem-A
Opal (on record for GIA, FTC, CIBJO; Gem-A unchecked — bodies without a recorded wording show the entry's own English name)
GSJ (Japan) formal name
蛋白石（オパール、タンパクセキ）
Chinese formal name
欧泊 (GB/T 16552—2017 Table A.2)
Mineralogical name
Natural opal; hydrated amorphous silica
Trade names and synonyms
Oolitic opal (narrower name (variety or subtype))

Four-language names

简体中文
欧泊
繁體中文
歐泊
English
Opal
日本語
オパール／蛋白石

National and regional formal names

Germany (RAL)
Opal
European Union
Opal
France (DGCCRF)
Opale; DGCCRF: origine et traitement à divulguer
Australia (GAA)
Opal; Australian Opal Code: disclose type, origin, treatment

Qualifiers stripped from the headline name

  • FTC: (§23.24: disclose treatments; §23.25: synthetic and imitation naming)
  • CIBJO: (CIBJO Gem Materials: disclose if treated/synthetic/assembled)

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
Natural amorphous gem material / mineraloid
Mineral chemical class
Not applicable
Construction or treatment status
Natural identity; treatment status unknown

Treatments listed by standards

GB/T Table B.1In the opal row of GB/T 16552—2017 Annex B Table B.1, filling for Opal improves or changes durability and appearance, with its category defined by grade, the grade definitions being on the treatments page; dyeing improves appearance and is classed as a treatment; coating improves or changes appearance such as lustre and colour and is classed as a treatment. JJA/AGL designationsThe Japanese JJA/AGL table also lists Water Opal under the species opal, colourless, impregnated with a clarifying agent (impregnation), with a note on play-of-colour; the JJA/AGL entries come from the main gemstone nomenclature table of the 2025 revision of the Definitions and Nomenclature of Gemstones and Ornamental Materials.

GB/T 16552-2017, Annex B, Table B.1

Table B.1, row “Opal” (p.16):

This table scrolls sideways on narrow screens.

MethodEffectCategory
FillingImproves or changes durability and appearanceGraded — see §5.4.1, Table 1
DyeingImproves appearanceTreatment
CoatingImproves or changes lustre, colour and other aspects of appearanceTreatment

What Table B.1 is, and how the “see 5.4.1” grades are defined: see the GB/T 16552: optimization vs treatment page.

JJA/AGL (Japan) table of principal gem names, 2025

This table scrolls sideways on narrow screens.

Gem nameSpeciesColourListed in the tableNotes
Water OpalOpalcolourlessan untreated tier is listed; impregnation with a colourless agent (Impregnated)play of colour; no dedicated entry on this site; shown on the broader entry

JJA/AGL definitions and nomenclature (2025 revision), table of principal gem names, p.9

How to read the JJA/AGL table: see the Gem treatments page.

What it is and how it's identified

Registered material facts for Opal: chemical composition is SiO2·nH2O, the phase is opal, the structure is amorphous, and the IMA nomenclature status is G, grandfathered (described before the IMA existed and generally regarded as a valid species). GB/T 16553—2017 gives Opal a Mohs hardness of 5–6, and the GIA Gem Encyclopedia gives 5–6.5; cleavage is none. Density is 1.25–2.23 g/cm³, derived by this site from the cited clause's value of 2.15 and its tolerance of +0.08/−0.90. Refractive index is 1.37–1.47, derived by this site from the cited clause's value of 1.450 and its tolerance of +0.020/−0.080; this site notes that the clause has finer sub-ranges and the overall value is taken. Optical character is isotropic, with anomalous extinction often seen in fire opal; birefringence is none, and untreated Opal shows no pleochroism. Among optical effects, play-of-colour can be seen, and chatoyancy can be seen but is rare. Untreated Opal has a vitreous to resinous lustre.

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”.

Definition

Opal — nomenclature status: IMA status G: grandfathered (described before the IMA existed, and generally regarded as a valid species).

CIBJO Gemmological Laboratories Book 2024

Source & check 2026-09-22
Source
Fact checked on
2026-09-22

Opal — composition (chemistry): SiO2 · nH2O

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Opal — composition (phase): opal

CIBJO Gemmological Laboratories Book 2024

Source & check 2026-09-22
Source
Fact checked on
2026-09-22

Structure

Opal — structure (overall): amorphous (not crystalline)

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Durability

Mohs hardness2 sources compared

12345678910GB/T 16553—20175–6GIA Opal5–6.5References (diamond = value, bar = range; below)

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))

Mohs hardnessSource: GB/T 16553—2017 · GIA Opal

Opal — Mohs hardness: 5–6

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Opal — Mohs hardness: 5–6.5

GIA Opal

Source & check 2026-09-20
Source
Fact checked on
2026-09-20

Opal — cleavage: none

GB/T 16553—2017

Does this standard apply
applies to this entry
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

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

In the source these care requirements sit on each opal variety row (black, white, fire and so on) and the species header row is blank; every listed variety row carries the same set, so we give it here under the general name.

Source & check 2026-09-21
Source
Fact checked on
2026-09-21

For Opal, the stability of irradiation is “unstable”.

CIBJO Gemstone Book 2022

Treatment state
treated
Treatment
irradiation
Source & check 2026-09-22
Source
Fact checked on
2026-09-22

Physical properties

Opal — density: 1.25–2.23 g/cm³

GB/T 16553—2017

0.51.52.53.54.55.56.57.51.25–2.23
Specific gravitySource: GB/T 16553—2017
Does this standard apply
applies to this entry
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

Optical effects on record: play of colourcause (schematic)chatoyancyrepresentative example: cause in Tiger’s Eye (schematic)

Opal — optic character: isotropic; fire opal commonly shows anomalous extinction.

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Opal — refractive index: 1.37–1.47

GB/T 16553—2017

1.31.72.12.52.93.31.37–1.47
Refractive indexSource: GB/T 16553—2017
Does this standard apply
applies to this entry
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

Opal — birefringence: none

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Opal — pleochroism: none

GB/T 16553—2017

Treatment state
untreated
Does this standard apply
applies to this entry
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

Opal — fluorescence (overall): black or white body colour: none to moderate, white to light blue; green, yellow; may be phosphorescent; black opal of other body colour: none to strong, green, yellow-green; may be phosphorescent; fire opal: none to moderate, green-brown; may be phosphorescent.

GB/T 16553—2017

UV fluorescencewhitelight bluenone–moderate, black or white body colourgreenyellowblack or white body colourgreenyellow-greennone–strong, black opal of other body colourgreen-brownnone–moderate, fire opalDots are illustrative colours built from the colour names, not measured values

Treatment state
untreated
Does this standard apply
applies to this entry
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

Opal — spectrum (UV-visible): green opal: 660 nm absorption peak; 470 nm absorption peak; others non-characteristic.

GB/T 16553—2017

400450500550600650700 nmgreen opal470660
Absorption spectrum (schematic)Line = absorption peak; hatching = absorption band; heavier or darker = higher recorded strength grade (not a measured intensity); Schematic: positions and strength grades as given by the cited source, not a measured curveSource: GB/T 16553—2017
Does this standard apply
applies to this entry
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

Opal — spectrum (infrared): The mid-IR spectrum shows the band pattern typical of opal

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Opal — optical effect (play of colour): seen

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Opal — optical effect (chatoyancy): seen; rare

GB/T 16553—2017

Does this standard apply
applies to this entry
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

Opal — magnification: colour patches with irregular, flaky outlines whose edges are flat and rather blurred, silky-looking surface, mineral inclusions.

GB/T 16553—2017

Treatment state
untreated
Does this standard apply
applies to this entry
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

body colour2 sources compared

Opal — body colour: Body colour varies. Play-of-colour opal with a white body colour may be called white opal; opal with a black, dark grey, blue, green, brown or other dark body colour may be called black opal; orange, orange-red and red opal may be called fire opal.

GB/T 16553—2017

Treatment state
untreated
Does this standard apply
applies to this entry
Qualification

The colours GB/T 16553—2017 lists for Synthetic Opal overlap with this entry: both give only “all body colours”.

General note
colour alone is not identity
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

Opal — body colour: white, grey, black, virtually any body colour.

CIBJO Gemmological Laboratories Book 2024

Treatment state
untreated
Qualification

In chapter 7 of the CIBJO Gemmological Laboratories Book, the colour descriptions of Howlite, Snowflake Obsidian, Onyx and other entries also name white or a whitish colour.

General note
colour alone is not identity
Source & check 2026-09-22
Source
Fact checked on
2026-09-22

How the sources relate

GB/T 16553—2017 and CIBJO Gemmological Laboratories Book 2024

complementary, no conflict

Both say opal can have virtually any body colour (the CIBJO Gemmological Laboratories Book names white, grey and black). GB/T 16553—2017 adds that, by body colour, it may be called white opal, black opal or fire opal.

Diffraction of visible light by monosized, well-ordered silica spheres is what generates play-of-color in precious opal.

Schematic of the cause of play of colour: well-ordered silica spheres
play of colourSchematic, not to scaleDrawn from the cause recorded under “Material research” on this page (source: G&G Fall 2013)If you reuse this image, please credit LumaLark Wiki and link to this pageLicense: CC BY 4.0
CIBJO definitions
play of colour, chatoyancy

Quality factors

Opal — quality factor (lustre): vitreous to resinous

GB/T 16553—2017

Treatment state
untreated
Does this standard apply
applies to this entry
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

Each row's “Source & check” expands to the full source; the date beside it is the fact check date.

1.451.51.822.2Refractive indexSpecific gravity12345Opal

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)1.41.51.61.71.81.922.12.22.32.41234567Refractive indexSpecific gravity12345Opal

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

Refractive index × specific gravity (all registered materials)Each dot is one material's registered refractive index and specific gravity (range midpoints, not measurements); gold = this material, dark = the nearest few (picked on this site by distance in RI ÷ 0.02 and SG ÷ 0.1, leaving out near-identical varieties of the same species)Source: each material's registered refractive index and specific gravity facts (GB/T 16553 and others; sources on each entry)

Testing and market

The tests the CIBJO laboratory protocol requires for this material, and what each method means, are in the protocol table on the laboratory testing page: §6.1.56.

How far testing can go

Opal that has undergone filling can be identified by the following features: Colourless plastic is injected and the density is low; infrared spectra show characteristic absorption bands of the filler.

GB/T 16553—2017

Treatment state
treated
Does this standard apply
can be followed as a reference only
Treatment
filling
Source & check 2026-09-21
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (filling) (locator checked 2026-09-21)
Fact checked on
2026-09-21

Opal that has undergone coating can be identified by the following features: Under magnification the surface luster is abnormal and the film can be seen peeling off in places; refractive index readings may be abnormal; infrared and Raman spectra show characteristic peaks of the film.

GB/T 16553—2017

Treatment state
treated
Does this standard apply
can be followed as a reference only
Treatment
coating
Source & check 2026-09-21
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (coating) (locator checked 2026-09-21)
Fact checked on
2026-09-21

Opal that has undergone dyeing can be identified by the following features: Under magnification the colour is unevenly distributed, mostly as fine particles concentrated in fissures, between grains or in surface hollows.

GB/T 16553—2017

Treatment state
colour treated
Does this standard apply
can be followed as a reference only
Treatment
dyeing
Source & check 2026-09-21
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.2.3 (dyeing) (locator checked 2026-09-21)
Fact checked on
2026-09-21

Opal — synthetic counterpart on the market: A synthetic counterpart of this material is commercially available

CIBJO Gemstone Book 2022

Treatment state
any treatment state
Source & check 2026-09-25
Source
Fact checked on
2026-09-25

For Opal in trade, the reported frequency of irradiation is “rarely”.

CIBJO Gemstone Book 2022

Treatment state
treated
Treatment
irradiation
Source & check 2026-09-22
Source
Fact checked on
2026-09-22

Opal — what the CIBJO laboratory protocol says to watch for: sugar and smoke treatments, dye, impregnation, synthetics.

CIBJO Gemmological Laboratories Book 2024

Treatment state
any treatment state
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

Opal — notes on test methods in the CIBJO laboratory protocol:

CIBJO laboratory protocol, in its own order; only the notes registered on this site
Test methodWhat the protocol notes for it
Phenomena: Play of colour, Cat’s-eye (rare), Star (rare);
UV-Visible(-NIR) spectroscopy: If dye is suspected

CIBJO Gemmological Laboratories Book 2024

Treatment state
any treatment state
Source & check 2026-09-26
Source
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.

Clause-level definitions

Opal has direct definitions in the CIBJO Gemmological Laboratories Book 2024. The row under the term Oolitic opal defines it as play of colour occurring together with numerous black or brown spherical inclusions; 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. The row under the term Opal registers it as mineral species opal, IMA status G, and the CIBJO wording is quoted in full under Clause-level definitions on this page; the cross-check shows it is defined directly by CIBJO and also appears in the GB/T annex. The IMA Mineral List also lists Opal, recording SiO2·nH2O and IMA status G, cross-checked against RRUFF and the IMA master mineral list, using public domain data.

This table scrolls sideways on narrow screens.

TermClauseDefinitionCross-check
Oolitic opal7.2.1.161Oolitic opal shows play-of-colour in combination with numerous black or brown spherical inclusions that have the appearance of fish eggs (roe) and are scattered throughout the stoneDefined directly by CIBJO; no identically named English entry was found in this edition of the GB/T annex.
Opal7.2.1.162white, grey, black, virtually any body colour, play-of-colour is often present. May show chatoyancyDefined directly by CIBJO; also present in the GB/T annex.
OpalIMA Mineral List entryOpal SiO2·nH2O G ?Cross-checked against the RRUFF/IMA master mineral list (public domain data).

Evidence and sources

Authority basis and clause sources

GB/T 16552—2017
Listed Table A.2 “Opal” · Source
IMA Mineral List 2026-07
Listed “Opal” · Source
CIBJO Gemmological Laboratories Book 2024
Listed §7.2.1.162 “Opal” · Source

Sources

Further reading

Related pages

Treatments recorded for this material

Recorded cultural contexts

Birthstone list

The U.S. Jewelers of America roster lists Opal for October; the U.K. National Association of Jewellers chart lists Opal as traditional for October; and Japan's JJA 2021 revised roster lists Opal for October. This records trade-association convention only; it does not establish material identity, natural origin, treatment, provenance, or effect.

Relation
Industry list inclusion

This table scrolls sideways on narrow screens.

Publisher or holderRegionFrom / periodSource and locator
Jewelers of AmericaUnited States1912 to the presentJewelers of America, “Birthstone Jewelry” (web page, consulted 2026-08-22)
Birthstone roster table, October row: Opal
Japan Jewellery AssociationJapanfrom 20 December 2021Tanjoseki (Japanese birthstone roster, Japan Jewellery Association page; revision of 20 December 2021)
2021 revised birthstone roster, October row: Opal
National Association of JewellersUnited Kingdom—National Association of Jewellers, “Zodiac Birthstones Jewellery: Star Signs, Gemstones and Their Meaning” (web page, consulted 2026-08-22)
Zodiac birthstone chart, October row: Opal (Traditional)

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 (Opal) is marked.

This table scrolls sideways on narrow screens.

PageContentStatus
Browsing familyOpal and natural amorphous silica materialsLive
Treatments recorded for this materialCoated, Filled / Fracture-filled, Dyed, ImpregnatedLive
Related articlesTo be added (placeholder)Planned

Who maintains this page

The material facts in the 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.

Facts editor
June
Reviewer
Ralph
How facts are checked
Each fact is matched to the clause it cites; facts checked between 2026-09-20 and 2026-09-26
Last revised
2026-10-12
Purpose
Lets buyers and sellers check naming, disclosure and material facts clause by clause.

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