GEMSTONE FIELD GUIDE
Tanzanite
Gem variety of a crystalline mineral. Blue-violet zoisite
The central tanzanite in the Smithsonian's Schneider Tanzanite Ring, NMNH G10184-00.
The violet-blue center stone is tanzanite; the smaller surrounding stones are diamonds. One photograph does not demonstrate pleochroism or establish treatment status.
Chip Clark / Smithsonian National Museum of Natural History, Schneider Tanzanite Ring, NMNH G10184-00 / CC0 · Source ↗ · CC0 1.0 Universal · resized for the webA verified 3D specimen capture is not available for this entry. No generated stand-in is used.
AT A GLANCE
Start with the evidence.
Tanzanite is the blue-to-violet gem variety of zoisite. Direction-dependent color is an important clue, but similar-looking manufactured materials exist. Identification requires measurements of the actual stone; a photograph is not a laboratory result.
- GIA — Tanzanite overview ↗
- Kiefert and Schmidt, GIA G&G (1996) — Some Tanzanite Imitations, pp. 270–276 ↗
- Refractive index (RI)
- 1.691–1.700
- Specific gravity (SG)
- About 3.35
- Mohs hardness
- 6–7
GIA tanzanite reference
How to read this value
Do not substitute MSA's wider all-zoisite mineral interval for this named gem-variety reference. A suitably polished facet, rotation and appropriate polarization are needed to investigate the RI range; one face-up value is not a complete test. Overlapping optical properties do not establish natural origin or treatment status.
GIA tanzanite reference
Tanzanite
These are reference properties, not measurements of the pictured specimen. Reference images are educational—not NumisTrends sale inventory.
See the details and limitations ↓01 · IDENTIFY
Build a profile, not a guess.
Tanzanite reference, not all colour varieties or compositions of zoisite.
Refractive index (RI)1.691–1.700
GIA tanzanite reference
Do not substitute MSA's wider all-zoisite mineral interval for this named gem-variety reference. A suitably polished facet, rotation and appropriate polarization are needed to investigate the RI range; one face-up value is not a complete test. Overlapping optical properties do not establish natural origin or treatment status.
Specific gravity (SG)About 3.35
GIA tanzanite reference
Mohs hardness6–7
Tanzanite
Birefringence0.008–0.013
GIA tanzanite reference
Not the same as pleochroism, and not necessarily obtainable in one viewing direction.
Optic characterBiaxial (+)
Zoisite / tanzanite
CleavagePerfect {010}; imperfect {100}
MSA zoisite mineral reference
Open a property for its scope and source. RI and specific gravity have no units. Mohs describes scratch resistance—not resistance to breaking. Do not scratch a stone to test it.
What can look similar?
Cobalt-bearing synthetic forsterite
Its violet appearance and strong pleochroism can resemble tanzanite.
How to investigate. GIA's 1999 samples had RI approximately 1.635–1.671, distinctly below tanzanite. Laboratory chemical and spectroscopic work supported the identification.
It is a different mineral used as an imitation, not laboratory-grown tanzanite.
Blue-violet glass
Colored glass can approximate tanzanite's face-up appearance.
How to investigate. Five heavy-glass imitations studied in 1996 were isotropic, with RI 1.600–1.605. The study also examined microscopic features and composition.
Those measurements belong to that glass product; other glass recipes differ. An apparently clean interior does not rule out glass.
02 · TESTS & ULTRAVIOLET
What a test can—and cannot—tell you.
Results depend on the specimen, instrument and method. This is a reference for understanding evidence, not a home authentication or treatment verdict.
UV reaction
- Long-wave UV
- One 3.46 ct pear-shaped tanzanite examined in GIA's 1999 Gem News showed weak, slightly chalky bluish-green fluorescence; three comparison tanzanites were inert.
- Short-wave UV
- That same 3.46 ct tanzanite showed no response to short-wave UV in the 1999 examination.
This is a specimen report, not a universal reaction or a natural-origin test. The note does not give numerical lamp wavelengths. Its authors could not establish how common the response was.
Professional refractive-index examination
Whether the stone's light-bending measurements are consistent with zoisite rather than a lookalike.
Reference observation. GIA's overview lists RI 1.691–1.700 and birefringence 0.008–0.013. These are reference values, not measurements of the pictured specimen.
Limit. Do not treat one reading as an identification. A gemologist selects suitable tests for the stone's size and transparency, then reconciles the results.
Pleochroism and optic-character checks
Directional color differences and optical behavior using professional gemological instruments.
Reference observation. Tanzanite is pleochroic and biaxial. These observations can help separate it from isotropic glass or uniaxial synthetic corundum.
Limit. Pleochroism is not unique to tanzanite; some imitations also show directional color. Confirm it alongside RI and other findings.
Real laboratory examples
Documented cases, not test results of the reference specimens shown on this page.
A fluorescent tanzanite challenged an absolute rule
In 1999, a 3.46 ct tanzanite fluoresced in long-wave UV despite the inert behavior of three comparison stones.
The takeaway. An unexpected UV response needs investigation, not automatic rejection of the identification.
Use a qualified gemologist for identification work. Do not expose eyes or skin to UV sources, or perform scratch, heat, acid or solvent tests on a gemstone. Do not assume that an absent UV reaction rules a material out.
03 · TREATMENTS & CARE
Protect the stone you actually have.
Disclosed treatments, fractures and settings can change which cleaning methods are appropriate.
Protect tanzanite from knocks and sudden temperature changes. Clean with warm soapy water; avoid ultrasonic and steam cleaning. Routine color-improving heat treatment is stable, but rare coatings or fracture filling require attention to the individual stone's treatment history.
04 · SOURCES & SCOPE
Know where each fact comes from.
Original NumisTrends explanations with primary references. This is not a replacement for gemological training, a laboratory report, or the examination of your own stone.
Properties reviewed: 2026-09-07 · Identification notes reviewed: 2026-09-07
NumisTrends is independent of GIA, Gem-A and the museums cited. A matching property cannot by itself establish identity, natural versus laboratory-grown origin, treatment, geographic origin, grade or value.