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Natural inorganic gem materials · Mineral / gemstone variety · Natural; common industry optimization (verify with certificate)

Tanzanite

The blue to violet gem variety of Zoisite, listed as "tanzanite" in GB/T 16552-2017 Annex A.1; heat treatment that turns brown material blue-violet is standard practice. 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 2026-10-13Who maintains this page

Care and wear

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

Tanzanite is not an independent mineral but the blue-to-violet gem variety of zoisite; its formation is essentially that of zoisite, crystallizing under medium-grade regional metamorphism within crystalline schists derived from calcium-rich protoliths, with the Merelani Hills of Tanzania its only commercial source worldwide. Its blue-to-violet appearance is dominated by strong three-directional pleochroism, and most gems on the market are heat treated to remove or reduce the yellow-green and brownish pleochroic colors, enhancing the blue and violet. This record does not describe tanzanite as a separate mineral.

Scope: Covers the identity, origin and cause of colour of tanzanite as a gem variety of zoisite; does not apply to treating tanzanite as an independent mineral species or assigning it a separate origin.

Sources GIA Tanzanite Description · GIA Tanzanite

Where it forms

The Merelani Hills of Tanzania (near Mount Kilimanjaro) — the world's only commercial source

Sources GIA Tanzanite

Zoisite's metamorphic setting: medium-grade regionally metamorphosed crystalline schists derived from calcium-rich protoliths

Sources GIA Tanzanite Description · GIA Tanzanite

How it forms, step by step

  1. Calcium-rich protoliths undergo medium-grade regional metamorphism to form zoisite-bearing crystalline schists.

    Sources GIA Tanzanite Description

  2. At the Merelani Hills, zoisite crystallizes into facetable transparent blue-to-violet crystals (tanzanite rough).

    Sources GIA Tanzanite

  3. The crystals show three directional pleochroic colors — red-violet, deep blue and yellow-green — owing to pleochroism.

    Sources GIA Tanzanite Description

  4. Most gems are heat treated to remove or reduce the yellow-green/brownish pleochroic color, enhancing the blue-violet appearance.

    Sources GIA Tanzanite · CIBJO Gemstone Book 2022

Link to appearance

Tanzanite's blue-to-violet appearance is dominated by pleochroism; cutters orient the stone to yield colors from violetish blue to bluish violet, with cool light (daylight-equivalent fluorescent) emphasizing blue and warm light (incandescent) making it appear more violet.

Sources GIA Tanzanite Description

Timescale

The sources state no geological timescale for tanzanite (zoisite) formation.

Sources GIA Tanzanite Description · GIA Tanzanite

Related base materials

  • Zoisite: Tanzanite is the blue to violet gem variety of zoisite; the mineral species is still zoisite, and tanzanite is not a separate mineral species

Supporting findings

Tanzanite (zoisite) occurs in the Merelani Hills of Tanzania and is the transparent variety of zoisite first unearthed there in 1967.

Evidence grade A

Scope: Describes tanzanite's occurrence and discovery; the formation mechanism belongs to zoisite.

Sources GIA Tanzanite

Recorded localities

Authoritative sources name a single or principal locality.

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

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

Tanzanite's color is dominated by pleochroism: in 1969 the American Mineralogist recorded red-violet, deep blue and yellow-green pleochroic colors; most gems are heat treated to remove or reduce yellow-green/brown to maximize blue and violet.

Evidence grade A

Scope: Describes the causal link between tanzanite's color, pleochroism and heat treatment; it makes no claim about other chromophores.

Sources GIA Tanzanite Description

How it is identified

Tanzanite's colour can be approximated by treated or imitation material, so appearance is insufficient to judge it. The distinction is made in a laboratory, where instruments test composition, spectra and internal features and thereby assess whether the stone is treated or an imitation; the conclusion should be issued by an accredited body.

Evidence grade A

Scope: Whether tanzanite is treated or an imitation is judged in a laboratory, with the conclusion from an accredited body.

Sources CIBJO Gemstone Book 2022

Other research findings

Tanzanite is the blue-to-violet gem variety of zoisite, not an independent mineral species, and is a commercial gem mined only in the Merelani Hills area of Tanzania.

Evidence grade A

Scope: Defines tanzanite as a zoisite variety; does not treat tanzanite as an independent mineral species.

Sources GIA Tanzanite Description · GIA Tanzanite · CIBJO Gemstone Book 2022

Tiffany & Co. chose the name tanzanite for the gem in honour of Tanzania, the country where it originated. The first crystal was found in 1967 and is credited to Ali Juuyawatu, a Masai tribesman.

Evidence grade A

Scope: Concerns naming and discovery history; it is not a formation conclusion.

Sources GIA Tanzanite

Most tanzanite on the market is heat treated, a normal and stable treatment; cobalt coating is a rare treatment.

Evidence grade A

Scope: Describes post-treatment of existing tanzanite; not laboratory synthesis of the crystal.

Sources GIA Tanzanite · CIBJO Gemstone Book 2022

Limits of this research

  • Research status: Formation, occurrence or other material-specific findings found in authoritative sources
  • The sources state no absolute geological timescale for tanzanite (zoisite) formation.
  • The specific chromophore of tanzanite's blue-to-violet color (e.g. V, Cr) is not explicitly attributed by the available sources; this record bases color cause only on pleochroism and heat treatment.

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
Tanzanite

GB/T 16552—2017 Table A.1 “Tanzanite” · IMA Mineral List 2026-07 “Zoisite”

Construction or treatment status

Natural; common industry optimization (verify with certificate)

Treatments recorded for this material: Heated / Heat-treated, Coated

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

Tanzanite is often subjected to heating. Under GB/T 16552-2017, heating counts as an optimization, so the plain gem name may be used (§5.4.2.1 a). Do not assume it is unenhanced.

Evidence

  • GB/T · Table A.1, row 坦桑石 (Tanzanite)

What this means for you

Before buying, check the recommended listing name "Tanzanite" 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

Tanzanite — care (cleaning): Warm soapy water is always safe; steam cleaning is never recommended.

GIA Gem Encyclopedia (care guide)

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

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

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

Gems & Gemology 1987 (Martin)

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

Tanzanite — care (heat): stable in normal wear, but keep it from very high temperatures and sudden temperature changes.

GIA Gem Encyclopedia (care guide)

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

care (chemicals)2 sources compared

Tanzanite — care (chemicals): Hydrochloric and hydrofluoric acids can attack it

GIA Gem Encyclopedia (care guide)

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

Tanzanite — care (chemicals): Coatings are often easily removed by solvents, heat or abrasives; keep it away from all solvents, heat and abrasives.

CIBJO Gemstone Book 2022

CIBJO Gemstone Book §6.2:special-care code n

Treatment state
treated
Only after these treatments
coating
Source & check 2026-09-25
Source
Fact checked on
2026-09-25

Tanzanite — care (chemicals): In the occasional fracture-filled stone, the filler may change in appearance over time.

GIA Gem Encyclopedia (care guide)

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

care (wear)2 sources compared

Tanzanite — care (wear): Mohs 6 to 7, fair to poor toughness and cleavage, so it breaks easily when struck; less hard and tough than sapphire, but if protected from rough wear and hard blows it suits any jewellery.

GIA Gem Encyclopedia (care guide)

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

Tanzanite — care (wear): cleaves or fractures easily or is brittle (e.g. rubbed facet edges); wear with care.

CIBJO Gemstone Book 2022

CIBJO Gemstone Book §6.2:special-care code b

Treatment state
treated
Only after these treatments
coating
Source & check 2026-09-25
Source
Fact checked on
2026-09-25

Tanzanite — care (wear): Occasional colour coatings do not last and may get scratched or damaged in ordinary wear.

GIA Gem Encyclopedia (care guide)

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

Tanzanite — care (wear): Routine heating turns a brownish colour blue to purple; the result is stable and brings no additional durability concern.

GIA Gem Encyclopedia (care guide)

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

Tanzanite — care (re-cutting): Coated stones are not suitable for re-cutting or re-polishing.

CIBJO Gemstone Book 2022

CIBJO Gemstone Book §6.2:special-care code n

Treatment state
treated
Only after these treatments
coating
Source & check 2026-09-25
Source
Fact checked on
2026-09-25

Tanzanite — care (special care): heated to violet-blue: special care (SC); avoid harsh abrasives.

AGTA Gemstone Information Manual, 17th ed.

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

Tanzanite — care (setting): prong and bezel: use caution; channel: use extreme caution; flush and pavé: avoid; note: one perfect cleavage; brittle and abrades easily.

Gems & Gemology 1987 (Martin)

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

Tanzanite — care (repair): pickle pot and polishing: use extreme caution; jeweller's torch and thermal shock: avoid; note: avoid heat, as it fuses under the torch.

Gems & Gemology 1987 (Martin)

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

Tanzanite — 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
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

CleaningTanzanite must not be cleaned ultrasonically (CIBJO Blue Book: any treatment state); GIA states that, in any treatment state, warm soapy water is always safe and steam cleaning is never recommended; Martin's Gemstone Durability states that, untreated, ultrasonic and steam cleaning should be avoided, as should thermal shock. Heat and chemicals: GIA states that, in any treatment state, Tanzanite is stable under normal wear but should not be exposed to very high temperatures or sudden temperature changes, and is attacked by hydrochloric and hydrofluoric acid; the CIBJO Blue Book states that, for coated stones, the coating is often easily removed by solvents, heat or abrasives, so they should be kept away from all solvents, heat and abrasives. Wear and setting: GIA states that, in any treatment state, Tanzanite (Mohs 6–7, fair to poor toughness, with cleavage) can be worn in all kinds of jewellery if protected and kept from rough wear and hard knocks; the CIBJO Blue Book states that coated stones break easily and should be worn with care; Martin's Gemstone Durability states that, untreated, heat, daylight and artificial light in display cases are relatively safe; prong and bezel settings call for caution, channel settings for particular caution, and tension and pavé settings should be avoided.

This table scrolls sideways on narrow screens.

QuestionMatching entries
Can it go in an ultrasonic or steam cleaner?Warm soapy water is always safe; steam cleaning is never recommended; care (cleaning); ultrasonic and steamer: avoid; note: avoid thermal shock
Heat and thermal shockstable in normal wear, but keep it from very high temperatures and sudden temperature changes; pickle pot and polishing: use extreme caution; jeweller's torch and thermal shock: avoid; note: avoid heat, as it fuses under the torch
Light and displayheat in the display case, sunlight and artificial light: relatively safe
Contact with acids, alkalis and household chemicalsHydrochloric and hydrofluoric acids can attack it; Coatings are often easily removed by solvents, heat or abrasives; keep it away from all solvents, heat and abrasives; In the occasional fracture-filled stone, the filler may change in appearance over time
Wearing limits (knocks, settings)Mohs 6 to 7, fair to poor toughness and cleavage, so it breaks easily when struck; less hard and tough than sapphire, but if protected from rough wear and hard blows it suits any jewellery; cleaves or fractures easily or is brittle (e.g. rubbed facet edges); wear with care; Occasional colour coatings do not last and may get scratched or damaged in ordinary wear; Routine heating turns a brownish colour blue to purple; the result is stable and brings no additional durability concern; prong and bezel: use caution; channel: use extreme caution; flush and pavé: avoid; note: one perfect cleavage; brittle and abrades easily

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: coating)

treated (Only after these treatments: filling)

treated (Only after these treatments: heating)

Durability and wearing

Toughness

Fair to poor toughness

Sources GIA Gem Encyclopedia (care guide)

Treatments and identification leads

Values not givenThe treatment recorded for Tanzanite is heat treatment, with status treated; the cited source gives no stability or prevalence, shown as “—”, which is not the same as “none”. Identification cluesThe identification lead for heat treatment is that some brownish zoisite turns violet-blue on heating, with a stable colour that is hard to detect. The source for heat treatment is the heat treatment entry in GB/T 16553—2017.

This table scrolls sideways on narrow screens.

TreatmentMarket frequency (CIBJO)Stability (CIBJO)GB/T 16552 categoryJJA/AGL listingIdentification leads
heatingStatus: treated——OptimizationheatingSee identification leads
Coating——Treatment——

Where each treatment acts (schematics): Heated / Heat-treated

“—” means the cited source gives no value for that item, not “none”. Source: GB/T 16553—2017; GB/T 16552—2017 Table B.1 (row for Zoisite); 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

A general LMHC wording rule also covers Tanzanite (Information Sheet #0): see the LMHC rule “What every report states”.

A general LMHC wording rule also covers Tanzanite (Information Sheet #8): see the LMHC rule “When colour authenticity cannot be determined”.

Tanzanite — how lab reports word it (definition used on reports): On reports, tanzanite is the vanadium-bearing blue to purple variety of zoisite.

LMHC Information Sheet #15

Definition in the information sheet (original wording): the vanadium-bearing blue to purple colour variety of the mineral zoisite

Case
when the variety name may be used
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

Tanzanite — how lab reports word it (definition used on reports): Other colours of zoisite are not “coloured tanzanite”; colour-zoned stones count only when blue-violet clearly dominates.

LMHC Information Sheet #15

Rule in the information sheet (original wording): These zoisite colour varieties should not be called tanzanites with a colour prefix.

Case
when the variety name may not be used
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

Tanzanite — how lab reports word it (optional comments): a reminder that heating is common and not always detectable

LMHC Information Sheet #15

Wording on the report: Tanzanite is commonly heated.

Case
heating is common but may be undetectable
Source & check 2026-09-26
Source
Fact checked on
2026-09-26

VerifyNames, standards, technical facts, clauses and sources

Names

Standard and listing namesThe Chinese formal name for Tanzanite comes from GB/T 16552—2017 Table A.1, and the suggested listing name is also Tanzanite. Trade and mineralogical namesThe mineralogical name describes a blue to violet gem variety of zoisite, the mineral species still being zoisite. Names in four languagesFor the four-language names, the Simplified and Traditional Chinese names are the same, the English name is Tanzanite, and a Japanese name is given. National standard namesAmong national and regional formal names, Germany (RAL) uses Tansanit, the European Union uses Tanzanite, noting that it is a geographical name, and France (DGCCRF) uses Tanzanite. Institution usageOn authority wording, the CIBJO wording is on record (CIBJO Gemmological Laboratories Book 2024), the FTC text contains no wording for this name (checked), and GIA and Gem-A are unchecked; bodies without a recorded wording show the entry's own English name, which does not represent their position. Japanese namesThe entry has its own Japanese name, and the wording used by GSJ (Japan) for the Japanese name of Tanzanite is not yet verified by this site.

Authority wording

GIA, FTC, CIBJO, Gem-A
Tanzanite (on record for CIBJO (CIBJO Gemmological Laboratories Book 2024); no wording for this name in the FTC text on file; GIA, Gem-A unchecked — bodies without a recorded wording show the entry's own English name)
GSJ (Japan) formal name
タンザナイト (not yet checked for this body — the entry's own Japanese name is shown)
Chinese formal name
坦桑石 (GB/T 16552—2017 Table A.1)
Mineralogical name
Blue to violet gem variety of zoisite; the mineral species is still zoisite

Four-language names

简体中文
坦桑石
繁體中文
坦桑石
English
Tanzanite
日本語
タンザナイト

National and regional formal names

Germany (RAL)
Tansanit
European Union
Tanzanite (EU: geographic name)
France (DGCCRF)
Tanzanite

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 / gemstone variety
Mineral chemical class
Silicates
Construction or treatment status
Natural; common industry optimization (verify with certificate)

Treatments listed by standards

GB/T Table B.1The zoisite (tanzanite) row of GB/T 16552—2017 Annex B Table B.1 lists for Tanzanite heat treatment (effect: improves colour; category: enhancement) and coating (effect: improves or changes appearance such as lustre and colour; category: treatment). JJA/AGL designationsIn the Japanese JJA/AGL table (2025), the gem name is Tanzanite, the species zoisite and the colour blue to violet-blue, with a separate no-treatment grade and heating (heat treatment) also listed. See alsoThe notes to Table B.1 and the related grade definitions are on the treatments page, as is how to read the JJA/AGL table.

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

Table B.1, row “Zoisite (Tanzanite)” (p.15):

This table scrolls sideways on narrow screens.

MethodEffectCategory
HeatingImproves colourOptimization
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
TanzaniteZoisiteblue to violetish bluean untreated tier is listed; heating (Heated / Heat-treated)

JJA/AGL definitions and nomenclature (2025 revision), table of principal gem names, p.14

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

What it is and how it's identified

The composition (phase) of Tanzanite is zoisite. The structural, durability, physical and optical data are values registered for zoisite (the whole mineral species), not measurements of Tanzanite itself: crystalline, orthorhombic; Mohs hardness 6–7; one direction of perfect cleavage; density 3.1–3.45 g/cm³ (clause value 3.35, tolerances +0.10 and −0.25, from which the limits are derived); optic character anisotropic, biaxial, positive; refractive index 1.686–1.705 (clause value 1.691–1.700, tolerance ±0.005, from which the limits are derived); birefringence 0.008–0.013. The UV-visible spectrum shows absorption peaks at 595nm and 528nm. For untreated body colour, GB/T 16553—2017 registers blue, violetish blue to bluish violet, and the CIBJO Gemmological Laboratories Book 2024 registers blue to bluish violet or bicoloured (green and blue); this site notes that the variety clauses for lazulite, axinite, sugilite and others in chapter 5 of GB/T 16553—2017, and entries such as sapphire, lazulite and Paraíba tourmaline in chapter 7 of the CIBJO Gemmological Laboratories Book, also give blue or bluish colours.

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 zoisite (the whole mineral species), so they are not repeated on the Tanzanite page; see the material facts on the Zoisite entry.

Items below that name the class they cover give the cited clause’s value for that whole class, not a measurement of Tanzanite (how to read class-level values).

Definition

Tanzanite — composition (phase): zoisite

CIBJO Gemmological Laboratories Book 2024

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

Structure

Tanzanite — structure (overall): crystalline

GB/T 16553—2017

Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Tanzanite — structure (crystal system): orthorhombic

GB/T 16553—2017

Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Tanzanite — structure (crystal habit): columnar or platy-columnar

GB/T 16553—2017

Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Durability

Tanzanite — Mohs hardness: 6–7

GB/T 16553—2017

123456789106–7References (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
Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Tanzanite — cleavage: perfect in one direction

GB/T 16553—2017

Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Physical properties

Tanzanite — density: 3.1–3.45 g/cm³

GB/T 16553—2017

0.51.52.53.54.55.56.57.53.1–3.45
Specific gravitySource: GB/T 16553—2017
Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Qualification

The clause we cite gives 3.35, with a stated tolerance of +0.10 / -0.25. The range shown here is worked out from that.

Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Optical

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

Tanzanite — optic character: biaxial, optically positive (anisotropic)

GB/T 16553—2017

Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Tanzanite — refractive index: 1.686–1.705

GB/T 16553—2017

1.31.72.12.52.93.31.686–1.705
Refractive indexSource: GB/T 16553—2017
Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Qualification

The clause we cite gives 1.691–1.700, with a stated tolerance of ±0.005. The range shown here is worked out from that.

Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Tanzanite — birefringence: 0.008–0.013

GB/T 16553—2017

00.050.10.150.20.250.30.008–0.013
BirefringenceSource: GB/T 16553—2017
Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Tanzanite — pleochroism: trichroic, strong; blue, purplish red and greenish yellow.

GB/T 16553—2017

Treatment state
untreated
Does this standard apply
applies to this entry
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 Pleochroism · Blue (tanzanite) (locator checked 2026-09-28)
Fact checked on
2026-09-26

Tanzanite — pleochroism: trichroism, strong. In blue tanzanite the three colours are blue, purple-red and green-yellow; brown stones show green, purple and light blue; yellow-green stones show dark blue, yellow-green and purple.

GB/T 16553—2017

Treatment state
untreated
Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

Tanzanite — spectrum (UV-visible): 595 nm absorption peak; 528 nm absorption peak.

GB/T 16553—2017

400450500550600650700 nm528595
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 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 UV-visible spectrum (locator checked 2026-09-28)
Fact checked on
2026-09-26

Tanzanite — optical effect (chatoyancy): seen; rare

GB/T 16553—2017

Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (locator checked 2026-09-26)
Fact checked on
2026-09-26

body colour2 sources compared

Tanzanite — body colour: blue, violet-blue to bluish violet

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 Lazulite, Axinite, Sugilite 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): §5.1.20 Colour · Tanzanite (locator checked 2026-09-28)
Fact checked on
2026-09-26

Tanzanite — body colour: blue to violet, or bi-colour (green and blue).

CIBJO Gemmological Laboratories Book 2024

Treatment state
untreated
Qualification

In chapter 7 of the CIBJO Gemmological Laboratories Book, the colour descriptions of Sapphire, Lazulite, Paraíba Tourmaline 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

How the sources relate

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

complementary, no conflict

Both give the range from blue to violet. The CIBJO Gemmological Laboratories Book also mentions bi-coloured stones (green and blue).

CIBJO definitions
chatoyancy

Quality factors

Tanzanite — quality factor (lustre): vitreous

GB/T 16553—2017

Treatment state
untreated
Does this standard apply
applies to this entry
What this covers
zoisite (the whole mineral species)
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (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.

1.671.681.691.71.711.723.13.23.33.43.5Refractive indexSpecific gravity12345Tanzanite

This material (RI / SG midpoints): 1.696 / 3.28; Closest values: 1Axinite 1.683 / 3.31, 2Hydrogrossular 1.7 / 3.35, 3Enstatite 1.712 / 3.31, 4Fei Cui / Jadeite Jade 1.683 / 3.35, 5Kornerupine 1.673 / 3.31

Show the full chart (330 registered materials)1.41.51.61.71.81.922.12.22.32.41234567Refractive indexSpecific gravity12345Tanzanite

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 identification lead for heated Tanzanite is that some brownish zoisite turns violetish blue on heating; the colour is stable and the treatment is difficult to detect. The heating lead registered in GB/T 16553—2017 covers zoisite as a whole mineral species and applies only to the treated state; GB/T 16553—2017 can be followed only as a reference for the Tanzanite entry and does not directly govern it. The CIBJO laboratory protocol covers Tanzanite under Zoisite: for identification see the Zoisite entry, and the list of required tests is on the laboratory testing page.

The CIBJO laboratory protocol covers Tanzanite under “Zoisite” (§6.1.92): see identification on the Zoisite entry; the required tests are listed on the CIBJO laboratory protocol table for Zoisite.

How far testing can go

Tanzanite that has undergone heating can be identified by the following features: Heating some brown-tinted zoisite produces a purple-blue colour; the colour is stable and the treatment is not easy to detect.

GB/T 16553—2017

Treatment state
treated
Does this standard apply
can be followed as a reference only
What this covers
zoisite (the whole mineral species)
Treatment
heating
Source & check 2026-09-26
Source
  • GB/T 16553—2017 Gems — Testing (GB/T): §5.1.20 (heating) (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.

Clause-level definitions

Tanzanite has a direct definition in the CIBJO Gemmological Laboratories Book 2024 under the term Tanzanite; the clause-level definition gives the mineral species as zoisite and the mineral or gemstone variety as tanzanite, and the CIBJO wording is quoted in full under Clause-level definitions on this page. The cross-check result for this entry is “defined directly by CIBJO; also appears in the GB/T annex”.

This table scrolls sideways on narrow screens.

TermClauseDefinitionCross-check
Tanzanite7.2.1.236blue to violet, or bi-colour (green and blue) variety of zoisiteDefined directly by CIBJO; also present in the GB/T annex.

Evidence and sources

Authority basis and clause sources

GB/T 16552—2017
Listed Table A.1 “Tanzanite” · Source
IMA Mineral List 2026-07
Listed “Zoisite” · Source
CIBJO Gemmological Laboratories Book 2024
Listed §7.2.1.236 “Tanzanite” · Source

Sources

Further reading

Related pages

Treatments recorded for this material

Recorded cultural contexts

Birthstone list

The U.S. Jewelers of America roster lists Tanzanite for December; the U.K. National Association of Jewellers chart lists Tanzanite as traditional for December; and Japan's JJA 2021 revised roster lists Tanzanite for December. 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 Statesfrom 2002Jewelers of America, “Birthstone Jewelry” (web page, consulted 2026-08-22)
Birthstone roster table, December row: Tanzanite
Japan Jewellery AssociationJapanfrom 20 December 2021Tanjoseki (Japanese birthstone roster, Japan Jewellery Association page; revision of 20 December 2021)
2021 revised birthstone roster, December row: Tanzanite
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, December row: Tanzanite (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.

This table scrolls sideways on narrow screens.

PageContentStatus
Browsing familyOther silicate mineralsLive
Treatments recorded for this materialHeated / Heat-treated, CoatedLive
Related articlesTo be added (placeholder)Planned

Who maintains this page

The material facts in the Tanzanite 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 2026-09-22 and 2026-09-26
Last revised
2026-10-13
Purpose
Lets buyers and sellers check naming, disclosure and material facts clause by clause.

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