On this page
UnderstandWhat it is, what people ask, how far this page goes
Natural inorganic gem materials · Mineral species · Natural identity; treatment status unknown
Iolite
Also known asCordierite
The gem name for the mineral species Cordierite. Natural blue stones are common, but synthetics and imitations still need to be ruled out. 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 Iolite 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 IoliteHow far lab testing can go and whether a synthetic exists
- Treatments and enhancements of IoliteThe enhancements and treatments the GB/T and JJA/AGL tables list
- Caring for and wearing IoliteCleaning, heat, chemicals and light, as the cited sources give them; durability indicators and wearing advice
- Hardness, density and refractive index of IoliteEach value with its source and check date
- Iolite 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?
- Name origin and history of IoliteWhere the name comes from
- Listing and disclosing IoliteThe listing name to use; what must be disclosed before sale
- Standard clauses that define IoliteWhich 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
Cordierite has the formula (Mg,Fe)2Al4Si5O18 and is orthorhombic (pseudohexagonal), commonly short prismatic and also compact massive. It forms mainly in thermally metamorphosed argillaceous sediments and in high-grade regionally metamorphosed schists, gneisses and granulites; it also occurs in mafic igneous rocks and granites, and as detrital grains. Cordierite is an independent mineral species; its blue gem variety is known as iolite, and the variety name is not an independent mineral species.
Scope: Covers the origin, composition and occurrence of cordierite as a mineral species in metamorphic and igneous rocks, and explains the relationship between the gem variety name iolite and the mineral name cordierite; does not address synthetic or imitation materials.
Sources Handbook of Mineralogy
Where it forms
Thermally metamorphosed argillaceous (aluminous) sediments
Sources Handbook of Mineralogy
Schists, gneisses and granulites of high metamorphic grade (regional)
Sources Handbook of Mineralogy
Mafic igneous rocks and granites
Sources Handbook of Mineralogy
How it forms, step by step
Aluminous (pelitic) sediments or protoliths undergo high-grade regional or contact metamorphism.
Sources Handbook of Mineralogy
Under suitable temperature–pressure conditions, Mg, Fe, Al and Si combine to crystallize (Mg,Fe)2Al4Si5O18 cordierite.
Sources Handbook of Mineralogy
Cordierite grows together with characteristic metamorphic minerals such as sillimanite, potassic feldspar, muscovite, biotite, garnet and andalusite.
Sources Handbook of Mineralogy
On weathering, cordierite may be released as detrital grains that concentrate in placer deposits.
Sources Handbook of Mineralogy
Link to appearance
Cordierite may be blue, smoky blue or bluish violet, also greenish, yellowish brown or grey, with a vitreous luster; its marked trichroic pleochroism (pale yellow-green, violet/blue-violet, pale blue) governs the stone's blue appearance and directional color variation.
Sources Handbook of Mineralogy
Timescale
The source states no absolute geological timescale for cordierite formation.
Sources Handbook of Mineralogy
Supporting findings
Cordierite forms mainly in thermally metamorphosed argillaceous sediments and in high-grade regionally metamorphosed schists, gneisses and granulites.
Evidence grade BScope: Describes cordierite's general metamorphic setting; it does not infer the detailed origin of any sample.
Sources Handbook of Mineralogy
Cordierite is commonly associated with sillimanite, potassic feldspar, muscovite, biotite, corundum, spinel, garnet and andalusite.
Evidence grade BScope: Describes common cordierite associations; not every locality contains all of these minerals.
Sources Handbook of Mineralogy
Recorded localities
Authoritative sources list the localities below; the list is not exhaustive.
Africa
- Kenya
Sources GIA Gem Encyclopedia
- Central Tanzania
Sources GIA Gem Encyclopedia
- MadagascarSignificant deposit discovered in 1994
Sources GIA Gem Encyclopedia
Asia
- Sri LankaGem gravels
Sources GIA Gem Encyclopedia · Gems & Gemology Winter 2000
- India
Sources GIA Gem Encyclopedia · Gems & Gemology Winter 2000
Europe
- Norway
Sources GIA Gem Encyclopedia
North America
- Slocan Valley, Selkirk Mountains, British Columbia, CanadaProduced in the 1990s
Sources Gems & Gemology Winter 2000
South America
- Brazil
Sources GIA Gem Encyclopedia
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
Cordierite is markedly pleochroic: X = pale yellow to green, Y = violet to blue-violet, Z = pale blue.
Evidence grade BScope: Describes cordierite's pleochroism as observed; it does not infer quantitative chromophore proportions.
Sources Handbook of Mineralogy
Where the name comes from
Cordierite commemorates the French geologist and mining engineer Pierre Louis A. Cordier (1777–1861), the first to study the species.
Evidence grade BScope: This states only the origin of the name and does not establish the material's composition, locality, natural origin, or whether it was treated.
Sources Handbook of Mineralogy
Other research findings
Cordierite has the formula (Mg,Fe)2Al4Si5O18 and is orthorhombic; it is dimorphous with indialite and forms a series with sekaninaite.
Evidence grade BScope: Describes cordierite's composition and polymorphic relations; not a compositional analysis of any specimen.
Sources Handbook of Mineralogy
Cordierite is an independent mineral species; iolite is its blue gem-variety name, and the variety name does not constitute an independent mineral species.
Evidence grade BScope: Defines the iolite-to-cordierite variety-name-to-species relationship; it does not treat a gem-variety name as an independent species.
Sources Handbook of Mineralogy
Limits of this research
- Research status: Formation, occurrence or other material-specific findings found in authoritative sources
- The source states no absolute geological timescale for cordierite formation.
- The source does not attribute cordierite's blue colour mechanism (e.g. Fe2+ charge transfer or a colour centre); this record therefore makes no chromophore attribution.
- The cited source is a mineralogical data entry and gives no P-T conditions or age for any specific deposit.
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
- Iolite (cordierite)
GB/T 16552—2017 Table A.1 “Iolite” · CIBJO Gemmological Laboratories Book 2024 §7.2.1.115 “Iolite”
Construction or treatment status
Natural identity; treatment status unknown
Related:Disclosure and cautions
This page cannot answer
Natural? · Origin · Price
A lab report settles these; this wiki covers naming and disclosure.
Related:What this page can answer
Answer at a glance
Naturally blue cordierite is common; synthetics and imitations must be excluded by testing.
What this means for you
Before buying, check the recommended listing name "Iolite (cordierite)" 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)3 sources compared
Iolite — care (cleaning): Some stones are susceptible to damage from ultrasonic cleaning. Do not expose them to ultrasonic cleaning.
CIBJO Gemstone Book 2022
CIBJO Gemstone Book §6.2:special-care code j
- Treatment state
- treated
- Only after these treatments
- dyeing
Source & check 2026-09-25
- Source
- CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §8 Annex C, Iolite, p.42; §6.2 j (locator checked 2026-09-25)
- Fact checked on
- 2026-09-25
Iolite — care (cleaning): ultrasonic and steamer: relatively safe.
Gems & Gemology 1987 (Martin)
- Treatment state
- untreated
- Qualification
The rating comes from the durability chart in Gems & Gemology (Summer 1987) and covers untreated stones only; it contradicts GIA’s current care advice for iolite, and both are shown.
Source & check 2026-09-26
- Source
- Martin, “Gemstone Durability: Design to Display”, Gems & Gemology, Summer 1987: pp.66–67 fold-out chart, Iolite row, CLEANING column (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Iolite — care (cleaning): Do not use steam or ultrasonic cleaning; warm soapy water works best (lukewarm water, mild soap and a toothbrush).
GIA Gem Encyclopedia (care guide)
- Treatment state
- any treatment state
- Qualification
GIA’s care page is current consumer advice (September 2026 snapshot) and does not separate treated from untreated stones; it contradicts the 1987 durability chart’s rating for untreated iolite, and both are shown.
Source & check 2026-09-26
- Source
- GIA Gem Encyclopedia — Iolite Care and Cleaning Guide: Section “Cleaning” (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
How the sources relate
- Gems & Gemology 1987 (Martin) and GIA Gem Encyclopedia (care guide)
The two sources say opposite things.
GIA’s iolite care page says to avoid steam and ultrasonic cleaning (warm soapy water works best) without separating treated from untreated stones, while the durability chart in Gems & Gemology (Summer 1987) rates both ultrasonic and steam cleaning of untreated iolite as relatively safe.
The two overlap on untreated iolite and contradict each other there; both are shown as published, without a ruling from LumaLark.
Iolite — care (heat): High heat or sudden temperature change can break it; its colour is stable.
GIA Gem Encyclopedia (care guide)
- Treatment state
- any treatment state
Source & check 2026-09-26
- Source
- GIA Gem Encyclopedia — Iolite Care and Cleaning Guide: Section “Stability” (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Iolite — 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
Source & check 2026-09-25
- Source
- CIBJO Blue Book — Official Gemstone Book (Coloured Stone Commission 2022) (CIBJO): §8 Annex C, Iolite, p.42; §6.2 k (locator checked 2026-09-25)
- Fact checked on
- 2026-09-25
Iolite — care (wear): perfect cleavage in one direction and modest toughness: a hard blow can split it, and extreme pressure or sharp temperature swings can break it.
GIA Gem Encyclopedia (care guide)
- Treatment state
- any treatment state
Source & check 2026-09-26
- Source
- GIA Gem Encyclopedia — Iolite Care and Cleaning Guide: Section “Hardness and Toughness” (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Iolite — care (special care): normal care
AGTA Gemstone Information Manual, 17th ed.
- Treatment state
- untreated
Source & check 2026-09-26
- Source
- AGTA Gemstone Information Manual, 17th edition (August 2026) (AGTA): p.X, Gemstone Information Chart, Iolite N, CARE REQUIRED / SPECIAL ADVICE (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
care (setting)2 sources compared
Iolite — care (setting): prong, bezel, flush, channel and pavé: use caution; note: one distinct cleavage.
Gems & Gemology 1987 (Martin)
- Treatment state
- untreated
Source & check 2026-09-26
- Source
- Martin, “Gemstone Durability: Design to Display”, Gems & Gemology, Summer 1987: pp.66–67 fold-out chart, Iolite row, SETTING column (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Iolite — care (setting): To lower the risk of breakage, jewellers prefer protective mountings or pieces that see little wear, such as pendants and earrings; cutting, polishing and setting need special care.
GIA Gem Encyclopedia (care guide)
- Treatment state
- any treatment state
Source & check 2026-09-26
- Source
- GIA Gem Encyclopedia — Iolite Care and Cleaning Guide: Section “Hardness and Toughness” (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Iolite — care (repair): polishing and thermal shock: use caution; jeweller's torch and pickle pot: avoid; note: heat may fuse it and acids attack it.
Gems & Gemology 1987 (Martin)
- Treatment state
- untreated
Source & check 2026-09-26
- Source
- Martin, “Gemstone Durability: Design to Display”, Gems & Gemology, Summer 1987: pp.66–67 fold-out chart, Iolite row, REPAIR column (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
Iolite — care (display): heat in the display case, sunlight and artificial light: relatively safe.
Gems & Gemology 1987 (Martin)
- Treatment state
- untreated
Source & check 2026-09-26
- Source
- Martin, “Gemstone Durability: Design to Display”, Gems & Gemology, Summer 1987: pp.66–67 fold-out chart, Iolite row, DISPLAY column (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.
Common care questions and matching entries
CleaningCleaning advice for Iolite differs by source and treatment status. CIBJOThe CIBJO Blue Book states that dyed iolite must not be cleaned ultrasonically, and that treatments with dye, oil, resin, wax or plastic are not permanent, so keep it away from all solvents, chemicals and heat. Martin, Gem DurabilityMartin's Gemstone Durability: Design to Display, covering only untreated stones, rates ultrasonic and steam cleaning as relatively safe, and states that thermal shock calls for caution, that the jeweller's torch and post-soldering pickling solution should be avoided, that heat, daylight and artificial light in display cases are relatively safe, and that prong, bezel, tension, channel and pavé settings call for caution. GIAGIA, regardless of treatment status, says not to use steam or ultrasonic cleaning and that warm soapy water is best; it states that high heat or sudden temperature changes may crack iolite, that the colour is stable, and that iolite has perfect cleavage in one direction and is not very tough, so a hard blow may split it and setting calls for special care; on cleaning, the original sources contradict each other.
This table scrolls sideways on narrow screens.
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.
untreated
- ultrasonic and steamer: relatively safe
- normal care
- prong, bezel, flush, channel and pavé: use caution; note: one distinct cleavage
- polishing and thermal shock: use caution; jeweller's torch and pickle pot: avoid; note: heat may fuse it and acids attack it
- heat in the display case, sunlight and artificial light: relatively safe
any treatment state
- Do not use steam or ultrasonic cleaning; warm soapy water works best (lukewarm water, mild soap and a toothbrush)
- High heat or sudden temperature change can break it; its colour is stable
- perfect cleavage in one direction and modest toughness: a hard blow can split it, and extreme pressure or sharp temperature swings can break it
- To lower the risk of breakage, jewellers prefer protective mountings or pieces that see little wear, such as pendants and earrings; cutting, polishing and setting need special care
treated (Only after these treatments: dyeing)
Durability and wearing
Toughness
Brittle
Fracture
Subconchoidal
Sources Handbook of Mineralogy
VerifyNames, standards, technical facts, clauses and sources
Names
Besides Iolite, the name register records "Cordierite", for which no restriction on use as a name is recorded; one further name in a non-Latin script is listed in the name register below (1 with no restriction recorded on use as a name).
Standard and listing namesThe Chinese standard name for Iolite is iolite (GB/T 16552—2017 Table A.1); the mineralogical name is Cordierite, the gem variety name is iolite, and the trade name or synonym is Cordierite. The suggested listing name is Iolite, based on GB/T 16552—2017 Table A.1 and the listing of Iolite in the CIBJO Gemmological Laboratories Book 2024. Names in four languagesNames are registered in four languages: Simplified Chinese, Traditional Chinese, English (Iolite) and Japanese. For the GSJ (Japan) formal name, the entry's own Japanese name is shown, and the wording used by the GSJ for this name is not yet verified by this site. Candidate search termsA transliteration of cordierite registered in the JJA/AGL gem name handbook 2025 is used only as a candidate search term. Institution usageOn authority wording, the wording of GIA and CIBJO is recorded as Iolite; FTC and Gem-A are unchecked, and bodies without a recorded wording show the English name of Iolite itself, which does not represent their position.
Name register
Every name recorded for this entry, with how it is classified, whether it may be matched to this entry automatically, whether it may be used as a product name, and the clause it rests on.
This table scrolls sideways on narrow screens.
| Name | Type | Matching | Use as a name | Basis |
|---|---|---|---|---|
| no restriction recorded (2) | ||||
| CordieriteEnglish | synonym | matched automatically | no restriction recorded | editorial ruling (no clause cited) |
| コーディエライトJapanese | transliteration | search candidate only | no restriction recorded | JJA/AGL Gemstone Nomenclature 2025 p.9 |
How each authority words this name: see Names › Authority wording.
- GSJ (Japan) formal name
- アイオライト／菫青石 (not yet checked for this body — the entry's own Japanese name is shown)
- Chinese formal name
- 堇青石 (GB/T 16552—2017 Table A.1)
- Mineralogical name
- Cordierite, gem variety iolite
- Trade names and synonyms
- Cordierite (synonym)
Four-language names
- 简体中文
- 堇青石
- 繁體中文
- 菫青石
- English
- Iolite
- 日本語
- アイオライト／菫青石
See also: Misleading and prohibited trade names · Supplementary name index
Classification
- Origin
- Natural materials
- Material domain
- Natural inorganic gem materials
- Browsing family
- Other silicate minerals
- Identity tier
- Mineral species
- Mineral chemical class
- Silicates
- Construction or treatment status
- Natural identity; treatment status unknown
Treatments listed by standards
In the table of principal gem names (p.11) of the JJA/AGL (Japan) definitions and nomenclature of gemstones and ornamental materials, November 2025 revision, Iolite is recorded with the gem name Iolite, species Iolite, and colours blue and dark brown, and the table lists an untreated tier as its entry. How to read the JJA/AGL table is explained on the Gem treatments 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 |
|---|---|---|---|---|
| Iolite | Iolite | blue, dark brown | an untreated tier is listed |
JJA/AGL definitions and nomenclature (2025 revision), table of principal gem names, p.11
How to read the JJA/AGL table: see the Gem treatments page.
What it is and how it's identified
Iolite has the chemical composition Mg2Al4Si5O18 and may contain Na, K, Ca, Fe, Mn and other elements as well as H2O; it is crystalline and orthorhombic, with short prismatic crystals and common twinning. Its Mohs hardness is 7–7.5, with one perfect cleavage. Density is 2.56–2.66 g/cm³, derived from the clause value 2.61 with a tolerance of ±0.05. Optically it is anisotropic, biaxial and negative; refractive index is 1.531–1.596, derived from the clause value 1.542–1.551 with a tolerance of +0.045/−0.011; birefringence is 0.008–0.012. It is transparent to translucent. The UV-visible spectrum shows weak absorption bands at 426nm and 645nm, and the mid-infrared region shows the characteristic infrared absorption bands of Iolite. Optical effects include chatoyancy and asterism, and aventurescence may be seen but is rare. Untreated, pleochroism is trichroic and strong: violet stones show light violet, dark violet and yellowish brown, and blue stones show colourless to yellow, bluish grey and dark violet; the lustre is vitreous.
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
Iolite — 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
Iolite — composition (chemistry): Mg2Al4Si5O18; Na, K, Ca, Fe, Mn and H2O can be present.
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — composition (phase): cordierite
CIBJO Gemmological Laboratories Book 2024
Source & check 2026-09-22
- Source
- Fact checked on
- 2026-09-22
Structure
Iolite — structure (overall): 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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — structure (crystal system): orthorhombic
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — structure (crystal habit): stubby prisms, often twinned
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Durability
Iolite — Mohs hardness: 7–7.5
GB/T 16553—2017
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))
- Does this standard apply
- applies to this entry
- General note
- hardness only, not durability
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — cleavage: perfect in one direction
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Physical properties
Iolite — density: 2.56–2.66 g/cm³
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- Qualification
The clause we cite gives 2.61, with a stated tolerance of ±0.05. The range shown here is worked out from that.
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Optical
Iolite — optic character: biaxial, optically negative (anisotropic)
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — refractive index: 1.531–1.596
GB/T 16553—2017
- Does this standard apply
- applies to this entry
- Qualification
The clause we cite gives 1.542–1.551, with a stated tolerance of +0.045 / -0.011. The range shown here is worked out from that.
Source & check 2026-09-21
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — birefringence: 0.008–0.012
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — transparency: transparent to translucent.
CIBJO Gemmological Laboratories Book 2024
Source & check 2026-09-22
- Source
- Fact checked on
- 2026-09-22
Iolite — pleochroism: strong trichroism. In purple stones: light purple, dark purple, yellowish brown; in blue stones: colourless to yellow, blue-grey, dark purple.
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
Source & check 2026-09-22
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.1.22 (locator checked 2026-09-22)
- Fact checked on
- 2026-09-22
Iolite — fluorescence (overall): 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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — spectrum (UV-visible): 426 nm weak absorption band; 645 nm weak absorption band.
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — spectrum (infrared): In the mid-IR, iolite shows its own diagnostic bands.
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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — optical effect (chatoyancy): 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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — optical effect (asterism): 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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — optical effect (aventurescence): 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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
Iolite — magnification: color band, gas-liquid inclusions, 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): §5.1.22 (locator checked 2026-09-21)
- Fact checked on
- 2026-09-21
body colour2 sources compared
Iolite — body colour: light to dark blue and violet.
CIBJO Gemmological Laboratories Book 2024
- Treatment state
- untreated
- Qualification
In chapter 7 of the CIBJO Gemmological Laboratories Book, the colour descriptions of Azurite, Benitoite, Colour-change Sapphire and other entries also name blue or a bluish colour.
- General note
- colour alone is not identity
Source & check 2026-09-22
- Source
- Fact checked on
- 2026-09-22
Iolite — body colour: pale to deep blue and purple; colorless, faintly yellowish white, green, grey, brown and other colors are also possible.
GB/T 16553—2017
- Treatment state
- untreated
- Does this standard apply
- applies to this entry
- Qualification
In chapter 5 of GB/T 16553—2017, the colour entries of Hemimorphite, Cat’s-eye, Sillimanite and other variety clauses also name blue or a bluish colour.
- General note
- colour alone is not identity
Source & check 2026-09-26
- Source
- GB/T 16553—2017 Gems — Testing (GB/T): §5.1.22 (locator checked 2026-09-26)
- Fact checked on
- 2026-09-26
How the sources relate
- CIBJO Gemmological Laboratories Book 2024 and GB/T 16553—2017
Both list light to dark blue and violet. GB/T 16553—2017 adds that colourless, slightly yellowish white, green, grey, brown and other colours may also occur. The CIBJO Gemmological Laboratories Book files this under the mineral name cordierite and notes that iolite is its gemmological name.
- CIBJO definitions
- chatoyancy, asterism, aventurescence
Quality factors
Iolite — quality factor (lustre): vitreous
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): §5.1.22 (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.
This material (RI / SG midpoints): 1.563 / 2.61; Closest values: 1Balin Stone 1.56 / 2.55, 2Scapolite 1.558 / 2.67, 3Synthetic Emerald 1.565 / 2.69, 4Synthetic Beryl 1.57 / 2.69, 5Amethyst 1.548 / 2.67
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 tests the CIBJO laboratory protocol requires for Iolite, and what each method covers, are given in the protocol table on the laboratory testing page. The CIBJO laboratory protocol's note on test methods gives the special optical effects as chatoyancy and the bloodshot effect. There is no synthetic Iolite on the 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: §6.1.43.
How far testing can go
Iolite — synthetic counterpart on the market: No synthetic counterpart of this material is commercially available
CIBJO Gemstone Book 2022
- Treatment state
- any treatment state
Source & check 2026-09-22
- Source
- Fact checked on
- 2026-09-22
Iolite — notes on test methods in the CIBJO laboratory protocol:
| Test method | What the protocol notes for it |
|---|---|
| Phenomena: | Cat’s-eye, bloodshot |
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
Iolite has clause-level definitions in the CIBJO Gemmological Laboratories Book 2024 under two terms, Cordierite and Iolite. Under Cordierite, it is defined as light to dark blue and violet, transparent to translucent, with a note that Iolite is the gemmological name for cordierite; the cross-check result for Cordierite is defined directly by CIBJO, with no English term of the same name found directly in the GB/T annex of this edition. Under Iolite, it is defined as the trade's use of iolite as a synonym for cordierite, violet to blue, transparent or translucent; the cross-check result for Iolite is defined directly by CIBJO, also appearing in the GB/T annex.
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| Term | Clause | Definition | Cross-check |
|---|---|---|---|
| Cordierite | 7.2.1.66 | light to dark blue and violet, transparent to translucent Comment Iolite is the gemmological term for cordierite (see Iolite) | Defined directly by CIBJO; no identically named English entry was found in this edition of the GB/T annex. |
| Iolite | 7.2.1.115 | In the trade, iolite is used as a synonym for cordierite; violet to blue, transparent, translucent | Defined directly by CIBJO; also present in 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.
- GIA – Gemological Institute of America (United States). Gem Encyclopedia: Iolite.
- RRUFF Project, in partnership with the IMA – International Mineralogical Association. IMA Mineral List.
Further reading
Recorded cultural contexts
Japan's JJA 2021 revised roster lists Iolite for March. This records trade-association convention only; it does not establish material identity, natural origin, treatment, provenance, or effect.
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| Publisher or holder | Region | From / period | Source and locator |
|---|---|---|---|
| Japan Jewellery Association | Japan | from 20 December 2021 | Tanjoseki (Japanese birthstone roster, Japan Jewellery Association page; revision of 20 December 2021) 2021 revised birthstone roster, March row: Iolite |
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.
This table scrolls sideways on narrow screens.
| Page | Content | Status |
|---|---|---|
| Browsing family | Other silicate minerals | Live |
| Related articles | To be added (placeholder) | Planned |
Who maintains this page
The material facts in the Iolite 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.