Is zoisite the same as tanzanite?
Tanzanite is the violet-blue variety of the mineral zoisite, hardness 6 to 7 (GIA). The IMA list has no tanzanite row; a green or pink zoisite is not tanzanite.
Only the blue ones. Tanzanite is not a separate mineral: GIA defines it as the violet blue to blue violet variety of zoisite, Ca2Al3(Si2O7)(SiO4)O(OH), hardness 6 to 7, specific gravity 3.35. A script over the September 2026 IMA list finds 0 lines containing tanzanite. Green, pink, yellow and gray zoisite should simply be called zoisite.
In short
- The colour-name rule. Call a stone tanzanite only when it is zoisite and violet blue to blue violet: GIA’s tanzanite page gives that definition, and the International Gem Society says non-blue zoisites, sometimes sold as “fancy color tanzanites”, should simply be called zoisites. Pink is thulite; green zoisite with ruby is anyolite. None of the three trade names has a row on the September 2026 IMA-CNMNC Master List.
- Same mineral, same numbers. GIA gives tanzanite specific gravity 3.35, hardness 6 to 7 and refractive index 1.691 to 1.700; the Handbook of Mineralogy zoisite sheet gives 3.15 to 3.36, 6 to 7 and α 1.685 to γ 1.725. A 20.00 g tanzanite reads 14.03 g in water, inside zoisite’s 13.65 to 14.05 g. Nothing in the numbers separates tanzanite from zoisite, because it is zoisite.
- Documented failure: the third-colour limit. Hurlbut (1969) recorded red-violet, deep blue and yellow-green pleochroism before heating, and violet-red with two deep blue directions after. GIA says heating removes or reduces the yellow-green or brownish colour. But Hurlbut also found untreated crystals with the heated pleochroism throughout, so a stone without the third colour is not proved heated.
- Heated, and at what temperature, depends on the source. GIA says most gems are heat treated; the gem society says to assume all blue to violet tanzanites have been. The 2006 IMA epidote-group report puts the brown-to-blue change at 400 to 500 °C, the gem society at about 500 to 600 °C. The two ranges meet only at 500 °C.
- Not sapphire, not iolite. The 1967 claim-holder hoped for a sapphire deposit (GIA), and Hurlbut says the crystals were first thought to be cordierite or dumortierite. At GIA’s 3.35, a 20.00 g tanzanite reads 14.03 g in water against at least 14.97 g for corundum on its sheet.
| Name on the label | What it is (source) | IMA row? | Colour (source) | Heat (source) | SG or density; 20.00 g piece in water | Choose this name when |
|---|---|---|---|---|---|---|
| Tanzanite | Violet blue to blue violet variety of zoisite (GIA) | No: 0 lines | Violet blue to bluish violet to violet purple (GIA) | Most gems heated (GIA); assume all (IGS) | 3.35 (GIA); 14.03 g | Zoisite that is violet-blue to blue-violet |
| Zoisite | The species (IMA; Handbook) | Yes: G, 1805, Austria | White, gray, greenish brown, greenish gray, pink, blue, purple (sheet) | Not stated for the species | 3.15 to 3.36; 13.65 to 14.05 g | Always correct, for any colour |
| “Fancy color tanzanite” | Green, yellow or pink zoisite sold under the tanzanite name (IGS) | No | Green, yellow, pink (IGS) | Not stated | Within zoisite’s range | Never: IGS says call it zoisite |
| Thulite | Pink zoisite (IGS); an obsolete name or synonym for zoisite cited by Dana 1896 (2006 report) | No: 0 lines | Pink (IGS) | Not stated | 3.09 (IGS); 13.53 g, below the sheet’s range | Pink massive material, with the species confirmed |
| Anyolite (ruby in zoisite) | Rock: chrome-rich green zoisite, black hornblende, opaque ruby (IGS) | No: 0 lines | Green with red (IGS) | Not stated | A mixture; not worked | The green and red lapidary rock |
| Clinozoisite | Same formula, monoclinic (IMA; sheet) | Yes: A, 2006 s.p., Austria | Colourless, pale yellow, pink, red, gray, green (sheet) | Not stated | 3.21 to 3.38; 13.77 to 14.08 g | Monoclinic material; never tanzanite |
| Sapphire | Corundum (GIA; corundum sheet) | Corundum: yes | Blue among others | Not compared here | 3.98 to 4.10; 14.97 to 15.12 g | Corundum, hardness 9; never zoisite |
Is zoisite the same as tanzanite? The criteria
Tanzanite is one colour variety of the mineral zoisite, so every tanzanite is a zoisite and most zoisites are not tanzanite. GIA’s tanzanite description calls tanzanite the violet blue to blue violet variety of the mineral zoisite, and says careful gemmological study determined that the 1960s crystals were a transparent variety of zoisite. The Handbook zoisite sheet gives the species Ca2Al3(SiO4)(Si2O7)O(OH), orthorhombic, hardness 6 to 7, density 3.15 to 3.36, with blue and purple among its colours and Merelani as a source of exceptional gem material.
What the IMA script finds. A case-insensitive scan of every line of the September 2026 list finds 0 lines containing tanzanite, 0 containing thulite and 0 containing anyolite, and 3 containing zoisite: zoisite, G, 1805, Austria; clinozoisite, A, 2006 s.p., Austria; and zoisite-(Pb), A, 2021-025, Sweden. The trade names are labels on the species, not species. GIA prints the chemistry as Ca2Al3(SiO4)3(OH), which has the same atoms as the IMA formula written another way.
Where the name came from. GIA’s tanzanite history page says that, as the most common story goes, in 1967 a Masai tribesman found violet-to-blue crystals at Merelani in northern Tanzania, and that Tiffany named the gem after the country and promoted it from 1968. Hurlbut (1969) records that the varietal name tanzanite had been suggested by Liddicoat and Crowningshield in 1968, and that the identity was established by X-ray photographs made independently at the University of Heidelberg and at the British Geological Survey and Natural History Museum. The species page zoisite covers the mineral; this page covers the word.
Is my tanzanite heated? The third-colour limit
Most tanzanite is heated, and pleochroism cannot prove it either way. GIA’s tanzanite description says most gems are heat treated, which removes or reduces the yellow green or brownish pleochroic colour, maximising the blue and violet. The International Gem Society tanzanite (zoisite) article says to assume all blue to violet tanzanites have received heat treatment and calls it a stable treatment. GIA’s gem treatment page says zoisite is often heated at low temperatures to remove a brownish colour component.
What Hurlbut measured. Before heating, X was red-violet, Y deep blue and Z yellow-green; after heating, X was violet-red and Y and Z both deep blue. But Hurlbut also found some untreated crystals that showed the deep blue colour and the same pleochroism as the heat-treated ones throughout, and others with it only towards the terminated end. So a missing yellow-green direction is consistent with heating and does not prove it; and since GIA says heating may only reduce that colour, its presence does not prove the opposite. That is the third-colour limit. Hurlbut attributes the colour to vanadium; the sheet’s analysis has 0.20% V2O3.
The temperatures disagree. Armbruster et al. 2006, Recommended nomenclature of epidote-group minerals (European Journal of Mineralogy 18, 551) says tanzanite turns from brown into blue upon heat-treatment at 400 to 500 °C. The gem society says tanzanites are typically heated to about 500 to 600 °C. Neither is a recipe and none is offered here. GIA also notes that tanzanites have rarely been coated to deepen their colour; coatings in general are on how to tell if a crystal is dyed.
Green, pink and gray zoisite: what to call them
Zoisite that is not violet-blue is zoisite, whatever the label says. The International Gem Society reports green, yellow and pink zoisites sold as “fancy color tanzanites”, and says that by definition tanzanites are blue to violet zoisites, heated or not, and non-blue zoisites should simply be called zoisites. Its faceted examples include a wine-yellow zoisite from Tanzania and a gray-green cushion from Pakistan.
Thulite and anyolite are names for zoisite in other colours. The gem society calls thulite pink, translucent to opaque zoisite, and anyolite, or ruby in zoisite, a lapidary rock of chrome-rich green zoisite, black hornblende and opaque ruby; its International Gem Society ruby article names Tanzania as the principal source of anyolite. The 2006 epidote-group report lists thulite and unionite among obsolete names or synonyms for zoisite cited by Dana in 1896. The gem society’s thulite figures, specific gravity 3.09 and biaxial (−), fall outside the zoisite sheet’s 3.15 to 3.36 and biaxial (+); a thulite label should be confirmed before it is read as zoisite.
Clinozoisite is never tanzanite. It has the same IMA formula but is monoclinic, and its pink and green crystals are a different species. Separating it from zoisite needs more than a balance: the shared in-water window is worked on zoisite and not repeated here. The epidote side of the family is on epidote.
Who each word is wrong for
Tanzanite is the wrong word for any zoisite that is not violet-blue to blue-violet, and for a mineral specimen from the Saualpe or Ducktown that happens to be zoisite. The Distribution field names many zoisite localities; GIA says tanzanite is mined commercially only in the Merelani Hills.
Zoisite alone is never wrong, but it can be incomplete. A label reading zoisite on a violet-blue Merelani crystal is correct; adding tanzanite records the variety. A label reading zoisite on pale prismatic material may be a determination to the zoisite–clinozoisite pair. An early label could read cordierite or dumortierite: Hurlbut says those were the first guesses for the Tanzanian crystals. The iolite side is on is cordierite the same as iolite and cordierite; dumortierite is on dumortierite.
Sapphire is the wrong word for tanzanite, and the balance shows it. At GIA’s 3.35, a 20.00 g tanzanite reads 14.03 g in water; corundum at 3.98 to 4.10 on its sheet reads 14.97 to 15.12 g, at least 0.94 g heavier, and it is hardness 9 on corundum. The method is on how to measure specific gravity at home; a stone in a setting cannot be weighed alone. Other identification questions are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.