Tanzanite Identification Guide
Identify blue-violet tanzanite by its dramatic pleochroism and properties, and distinguish it from sapphire, iolite, and imitations.
Read the full Tanzanite encyclopedia entry →
What Tanzanite Looks Like
Tanzanite is the blue-to-violet gem variety of the mineral zoisite, colored by vanadium. Almost all tanzanite is heat-treated to produce its signature blue; untreated stones are often brownish.
- Color: blue, violet-blue, to purple; strongly trichroic.
- Luster: vitreous.
- Transparency: transparent.
- Crystal habit: prismatic, striated orthorhombic crystals; gems are usually faceted.
Step-by-Step Field ID Checklist
- Rotate the stone. Tanzanite shows dramatic trichroism—blue, violet/purple, and burgundy-to-brownish flashes from different directions. This is its hallmark.
- Assess the body color. Rich blue with violet undertones.
- Hardness. Mohs 6-6.5—softer than sapphire; it can be scratched by quartz/topaz, so check for wear on facet edges.
- Cleavage caution. Tanzanite has perfect cleavage in one direction and is brittle—chips along edges are common.
- Streak. White (do streak tests only on rough).
- Inclusions. Often quite clean; natural inclusions differ from gas bubbles in glass.
Key Diagnostic Tests
- Hardness: 6-6.5.
- Streak: white/colorless.
- Cleavage: perfect in one direction (fragile).
- Pleochroism: strong trichroism—the decisive identifier.
- Density: ~3.35 (heavier than quartz, lighter than sapphire ~4.0).
- Refractive index/birefringence: moderate; a refractometer reading distinguishes it from many look-alikes.
Common Look-Alikes
- Blue sapphire: much harder (9), denser (~4.0), and shows weaker dichroism (not three colors); sapphire resists scratching far better.
- Iolite (cordierite): also strongly trichroic blue-violet but lighter (~2.6), with grayish-blue and yellowish pleochroic colors and lower RI; iolite's violet is softer/grayer than tanzanite's vivid blue.
- Blue spinel: singly refractive (no pleochroism), harder (8).
- Glass/synthetic imitations: no pleochroism, may show gas bubbles and warm feel; tanzanite's strong trichroism gives it away.
- Tanzanite-color CZ/forsterite simulants: differ in RI and density.
Where It Is Found
Tanzanite is famously single-source: the Merelani Hills near Arusha, Tanzania, in graphite-bearing metamorphic rocks. No other commercial deposit exists, which is part of its rarity and appeal.
Frequently asked questions
How can you tell if tanzanite is real?
Genuine tanzanite shows strong trichroism—turning it reveals blue, violet, and reddish-brown flashes from different angles. Combined with Mohs 6-6.5 hardness, specific gravity near 3.35, and one perfect cleavage, this confirms the stone.
What is the difference between tanzanite and sapphire?
Sapphire is much harder (9 vs 6.5) and denser, and shows only two-color dichroism, while tanzanite is softer, has perfect cleavage, and displays three distinct pleochroic colors.
Tanzanite vs iolite—how do you tell them apart?
Both are trichroic blue-violet, but iolite is lighter and shows grayer, more washed-out violet and yellowish tones, whereas tanzanite shows vivid blue and a richer purple with higher density and refractive index.
Is tanzanite heat treated?
Almost always. Most rough is brownish and is heated to about 500-600 degrees Celsius to develop the blue-violet color, which is a permanent and accepted treatment.
Tanzanite identified by the community
Recent Tanzanite specimens identified with Rock Identifier.
