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
Common Opal
Also known asCacholong Opal · Green Opal (conditional name) · Potch · Prase Opal
Opal that does not show play-of-color; even if it has body color it cannot be called precious opal. Impregnation treatment must be disclosed. Also known as Potch. 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 Common 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 Common OpalIdentification points from the research literature; how far lab testing can go and whether a synthetic exists
- Treatments and enhancements of Common Opal1 registered treatments: status, stability, how common they are and identification leads
- Caring for and wearing Common OpalCleaning, heat, chemicals and light, as the cited sources give them
- Hardness, density and refractive index of Common OpalEach value with its source and check date
- Common Opal 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 Common OpalThe listing name to use; what must be disclosed before sale
- Standard clauses that define Common 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
Common opal is natural opal in which play-of-color is absent. It is hydrated amorphous silica, as precious opal is, but the silica spheres inside it lack regular packing—their shapes are nonspherical and their sizes vary widely—so no periodic structure capable of producing play-of-color can arise. Diffuse scattering of light by these randomly placed nanospheres is what gives common opal its milky or hazy look (opalescence). The bodycolour depends largely on which nano-inclusions occur: hematite yields yellow, orange or brown, fluorite can yield purple, and nickel-rich inclusions can yield green.
Scope: Applies to natural common opal that does not show play-of-colour; does not apply to precious opal or man-made opal.
Sources GIA Opal Description · G&G Fall 2013 · G&G Summer 2025
Where it forms
A formation setting like other natural opal (silica-bearing groundwater depositing in rock cracks and between layers)
Sources G&G Fall 2013 · G&G Summer 2025
How it forms, step by step
Natural opal deposits from silica-bearing water and consolidates as it loses water into hydrated amorphous silica.
The silica spheres are nonspherical with broad size distributions and pack without order, so no 3D periodic structure forms.
The randomly arranged nanospheres scatter light diffusely, giving common opal a milky/hazy opalescent look; bodycolor is controlled by the species of nano-inclusions.
Link to appearance
Lacking an ordered sphere array, common opal produces no play-of-color; its appeal comes from the opalescent diffuse scattering and the bodycolor imparted by nano-inclusions.
Sources G&G Fall 2013 · G&G Summer 2025
Timescale
The cited sources give no absolute timescale for common opal formation.
Sources G&G Fall 2013 · G&G Summer 2025
Supporting findings
Common opal is likewise hydrated amorphous silica formed by silica-gel solidification, only with spheres that are not regularly stacked.
Evidence grade AScope: General natural-opal formation, including common opal.
Causes of colour and optical effects
Common opal does not show play-of-color usually because its silica spheres lack regular packing; the randomly arranged nanospheres scatter light diffusely, giving a milky/hazy appearance (opalescence).
Evidence grade AScope: A formation/optical-mechanism statement; ‘usually’ allows for exceptions.
Which nano-inclusions are present largely determines the bodycolour of common opal: hematite yields yellow, orange or brown; fluorite is able to yield purple; nickel-rich inclusions can yield green.
Evidence grade AScope: A general rule for coloring inclusions; not an exhaustive list of colorants.
How it is identified
Common opal lacks play-of-colour and is easily confused with other materials by sight; colour and transparency do not prove naturalness or treatment. Deciding whether a specimen is natural, laboratory-made or an imitation/assembled material requires laboratory measurement of its gemological properties and composition. The conclusion should be issued by a qualified gem laboratory and cannot be decided from appearance alone.
Evidence grade AScope: Applies where natural, synthetic and imitation opal must be distinguished; no self-administered criteria are given.
Other research findings
Common opal is natural opal without play-of-color, which is how it is distinguished from precious opal, which shows it.
Evidence grade AScope: The gemological split between precious and common opal; it is not a formation definition.
Sources GIA Opal Description · G&G Fall 2013
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 common opal formation and do not list all coloring-inclusion species.
- ‘No play-of-color’ in common opal has exceptions (some opals with regular structure still show none because sphere diameters are too large or too small to diffract visible light).
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
- Common Opal (without play-of-colour)
IMA Mineral List 2026-07 “Opal” · CIBJO Gemmological Laboratories Book 2024 §7.2.1.65 “Common 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
Common Opal is also known by several other names, some of which can refer to different materials in other contexts; use "Common Opal (without play-of-colour)" to avoid ambiguity.
Evidence
- CIBJO · 7.2.1.65
What this means for you
Before buying, check the recommended listing name "Common Opal (without 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 Common Opal, is set out in full under care on the Opal entry.
Care
Common Opal — care (light): may fade or revert to its original colour in strong light; do not wear or leave it in strong light for extended periods.
CIBJO Gemstone Book 2022
CIBJO Gemstone Book §6.2:special-care code f
- Treatment state
- any treatment state
- Colours covered
- blue to bluish-green common opal only
Source & check 2026-09-25
- Source
- CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §8 Annex C, Common Opal, p.65; §6.2 f (locator checked 2026-09-25)
- Fact checked on
- 2026-09-25
Common 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
- impregnation
Source & check 2026-09-25
- Source
- CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §8 Annex C, Common Opal, p.64-65; §6.2 k (locator checked 2026-09-25)
- Fact checked on
- 2026-09-25
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
Care for Common Opal, as registered from CIBJO: on light, in any treatment state, only blue to bluish green Common Opal may fade or revert to its original colour in strong light, so do not wear or leave it in strong light for extended periods; on chemicals, for treated Common Opal, limited to impregnated material, treatments with dye, oil, resin, wax or plastic are not permanent, so keep it away from all solvents (including dish-washing liquids), chemicals and heat.
This table scrolls sideways on narrow screens.
| Question | Matching entries |
|---|---|
| Light and display | blue to bluish-green common opal only: may fade or revert to its original colour in strong light; do not wear or leave it in strong light for extended periods |
| Contact with acids, alkalis and household 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 |
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
- blue to bluish-green common opal only: may fade or revert to its original colour in strong light; do not wear or leave it in strong light for extended periods §6.2 f
treated (Only after these treatments: impregnation)
Treatments and identification leads
Registered treatmentsThe registered treatment for Common Opal is impregnation, with the status treated, stability unstable and prevalence rare. Values not givenItems marked “—” in the treatment record mean the cited source gives no value, not “none”. SyntheticsAccording to the registered entries, synthetic Common Opal is common on the market, registered for any treatment state.
This table scrolls sideways on narrow screens.
| Treatment | Market frequency (CIBJO) | Stability (CIBJO) | GB/T 16552 category | JJA/AGL listing | Identification leads |
|---|---|---|---|---|---|
| impregnationStatus: treated | rarely | unstable | — | — | — |
| Filling | — | — | Graded — see §5.4.1, Table 1 | — | — |
| Dyeing | — | — | Treatment | — | — |
| Coating | — | — | Treatment | — | — |
Where each treatment acts (schematics): Impregnated
“—” means the cited source gives no value for that item, not “none”. Source: CIBJO Gemstone Book 2022; GB/T 16552—2017 Table B.1 (row for Opal); 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
VerifyNames, standards, technical facts, clauses and sources
Names
Besides Common Opal, the name register records "Cacholong Opal", "Green Opal", "Potch" and "Prase Opal", for which no restriction on use as a name is recorded (CIBJO Gemstone Book 2022 Annex 8 “Cacholong Opal”; CIBJO Gemstone Book 2022 Annex 8 “Green Opal”; CIBJO Gemstone Book 2022 Annex 8 “Prase Opal”).
Chinese standard nameThe national-standard naming counterpart for Common Opal is opal (GB/T 16552—2017 Table A.2); the name Common Opal is not listed in the text. Trade and mineralogical namesThe mineralogical name is Opal without play-of-colour, and the trade names and synonyms include Potch (Australian/trade term). Names in four languagesThe four-language names cover Simplified Chinese, Traditional Chinese, English (Common Opal) and Japanese. Recommended listing nameThe suggested listing name is "Common Opal (no play-of-colour)". Institution usageOn authority wording, CIBJO lists Common Opal (CIBJO Gemmological Laboratories Book 2024); the FTC text has no wording for this name (checked); 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 namesThe GSJ (Japan) formal name shows the Japanese transliteration of common opal; the GSJ's wording for this name has not yet been verified by this site, and the entry's own Japanese name is shown. Candidate search termsIn the name register, Green Opal serves only as a candidate search term.
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 (4) | ||||
| Cacholong OpalEnglish | narrower name (variety or subtype) | matched automatically | no restriction recorded | CIBJO Gemstone Book 2022 Annex 8 “Cacholong Opal” |
| Green OpalEnglish | narrower name (variety or subtype) | search candidate only | no restriction recorded | CIBJO Gemstone Book 2022 Annex 8 “Green Opal” |
| PotchEnglish | market name | matched automatically | no restriction recorded | editorial ruling (no clause cited); Note: Australian / trade term. |
| Prase OpalEnglish | narrower name (variety or subtype) | matched automatically | no restriction recorded | CIBJO Gemstone Book 2022 Annex 8 “Prase Opal” |
How each authority words this name: see Names › Authority wording.
- GSJ (Japan) formal name
- コモンオパール (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 without play-of-colour
- Trade names and synonyms
- Potch (market name) · Cacholong Opal (narrower name (variety or subtype)) · Prase Opal (narrower name (variety or subtype)) · Green Opal (narrower name (variety or subtype); conditional name)
Four-language names
- 简体中文
- 普通欧泊
- 繁體中文
- 普通歐泊
- English
- Common Opal
- 日本語
- コモンオパール
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
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.
JJA/AGL (Japan) table of principal gem names, 2025
This table scrolls sideways on narrow screens.
| Gem name | Species | Colour | Listed in the table | Notes |
|---|---|---|---|---|
| Common Opal | Opal | an untreated tier is listed | no play of colour |
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
The composition (phase) of Common Opal is opal, and its structure (overall) is amorphous. For structure, cleavage, density, optical character, refractive index, birefringence, pleochroism, optical effects and lustre, the cited clauses are written for opal (the whole category) and are not data measured on common opal alone. Mohs hardness is 5–6.5; the cited clause covers the whole species, and common opal is only one variety within the opal species; there is no cleavage. Density is 1.25–2.23 g/cm³, derived from the clause value of 2.15 with an allowed deviation of +0.08/−0.90; the refractive index is 1.37–1.47, derived from the clause value of 1.450 with an allowed deviation of +0.020/−0.080, the clause also having finer sub-ranges, with the overall value taken. It is optically isotropic, with no birefringence; for opal as a whole category, play-of-colour may be seen. In untreated common opal there is no pleochroism, and the body colour shows no play-of-colour; bluish-white, porcelain-like material may be called cacholong opal, and prase opal is green; a qualifier on body colour notes that the colour descriptions of other entries in Chapter 7 of the CIBJO Gemmological Laboratories Book also mention white or whitish tones; the lustre is vitreous to resinous.
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 Common 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 Common Opal (how to read class-level values).
Definition
Common Opal — composition (phase): opal
CIBJO Gemmological Laboratories Book 2024
Source & check 2026-09-22
- Source
- Fact checked on
- 2026-09-22
Structure
Common 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
Common 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
Common 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
Common 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 source lists blue to bluish-green common opal on a separate row which adds that the colour fades in strong light; that row is outside what this entry cites.
Source & check 2026-09-21
- Source
- Fact checked on
- 2026-09-21
For Common Opal, the stability of impregnation is “unstable”.
CIBJO Gemstone Book 2022
- Treatment state
- treated
- Treatment
- impregnation
Source & check 2026-09-25
- Source
- Fact checked on
- 2026-09-25
Physical properties
Common 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
Common 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
Common 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
Common 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
Common Opal — transparency: translucent to opaque.
CIBJO Gemmological Laboratories Book 2024
Source & check 2026-09-22
- Source
- Fact checked on
- 2026-09-22
Common 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
Common 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
Common 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
Common Opal — body colour: translucent to opaque opal without play-of-colour. Bluish white, porcelain-like common opal may be called cacholong opal; prase opal is green and moss opal is common opal with inclusions that look like moss.
CIBJO Gemmological Laboratories Book 2024
- Treatment state
- untreated
- Qualification
In chapter 7 of the CIBJO Gemmological Laboratories Book, the colour descriptions of Amazonite, Larimar, Lazulite and other entries also name white or a whitish colour.
- General note
- colour alone is not identity
Source & check 2026-09-26
- Source
- Fact checked on
- 2026-09-26
- CIBJO definitions
- play of colour, chatoyancy
Quality factors
Common 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
Related CIBJO terms
- Opalescence
the milky or pearly appearance of some gemstones, (especially some common opals).
CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §5.57, p.26
Definitions are quoted verbatim from the cited CIBJO Blue Book; […] marks an excerpt, and the full definition is at the cited clause.
Testing and market
Identification of Common Opal falls under the Opal line of the CIBJO laboratory protocol; see the identification section of the Opal entry, and the list of required methods on the laboratory testing page. Synthetic counterparts of Common Opal are common on the market, in any treatment state. For impregnated Common Opal, the reported frequency of impregnation in trade is “rarely”, which applies only to treated Common Opal whose treatment is impregnation.
The CIBJO laboratory protocol covers Common 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
Common Opal — synthetic counterpart on the market: Synthetic counterparts of this material are common in the trade
CIBJO Gemstone Book 2022
- Treatment state
- any treatment state
Source & check 2026-09-25
- Source
- Fact checked on
- 2026-09-25
For Common Opal in trade, the reported frequency of impregnation is “rarely”.
CIBJO Gemstone Book 2022
- Treatment state
- treated
- Treatment
- impregnation
Source & check 2026-09-25
- Source
- Fact checked on
- 2026-09-25
Each row's “Source & check” expands to the full source; the date beside it is the fact check date.
Clause-level definitions
Common Opal is defined directly in CIBJO Gemmological Laboratories Book 2024 under the entry name Common opal, with the definition mineral species opal, mineral/gem variety common opal, and the CIBJO wording quoted in full under Clause-level definitions on this page. The cross-check result is that it is defined directly by CIBJO, and no identically named English entry was found in the GB/T annex.
This table scrolls sideways on narrow screens.
| Term | Clause | Definition | Cross-check |
|---|---|---|---|
| Common opal | 7.2.1.65 | translucent to opaque opal without play-of- colour. Bluish white, porcelain-like common opal may be called cacholong opal; prase opal is green and moss opal is common opal with inclusions that look like moss | Defined directly by CIBJO; no identically named English entry was found in this edition of the GB/T annex. |
Evidence and sources
Authority basis and clause sources
Sources
- CIBJO – The World Jewellery Confederation. Gemmological Laboratories Book (Blue Book), Gemmological Commission 2024-1.
- CIBJO – The World Jewellery Confederation. The Gemstone Book (Blue Book), 2022.
- SAMR / SAC (China). GB/T 16552—2017 Gems — Nomenclature.
- RRUFF Project, in partnership with the IMA – International Mineralogical Association. IMA Mineral List.
- Geological Survey of Japan, AIST (Japan). Museum mineral specimens, browse by classification.
Further reading
Related entries and topic cluster
Parent entry and related pages on this site. Rows marked “Planned” are suggested new content and have no link yet.
Related-entry tree for Opal on this site: 17 entries listed by this site's parent-entry links; synthetic, treated and similar entries carry their recorded state after the name; this page (Common Opal) is marked.
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 Common 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.