How to identify Tanzanite

To correctly identify Tanzanite (Blue Zoisite), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically blue to violet-blue, often with a purplish tint. Exhibits strong pleochroism, showing blue, violet, and sometimes reddish-brown hues depending on the viewing angle.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Crystalline, typically found as prismatic crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthorhombic system, typically forming elongated, striated prismatic crystals. Crystals can be euhedral to subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect on {010}, good on {100}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks, specifically within graphitic gneisses and schists, often in association with calcium-silicate rocks. Found in veins and pods within these host rocks.

Key Facts

Hardness

6.0-7.0 on the Mohs scale

Specific Gravity

3.35 (+0.10, -0.05)

Crystal System

Orthorhombic

Color

Blue, violet, purplish-blue (pleochroic)

Luster

Vitreous

Transparency

Transparent to translucent

Fracture

Uneven to conchoidal

Cleavage

Perfect on {010}, good on {100}

Composition

Calcium aluminum hydroxyl sorosilicate, Ca2Al3(SiO4)3(OH)

Physical characteristics

• Crystal Habit: Prismatic, often striated parallel to the c-axis; massive, granular.
• Cleavage Type: Perfect on one direction ({010}), good on another ({100}). This makes it somewhat fragile and susceptible to chipping.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous.

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