How to identify Iolite

To correctly identify Iolite (Cordierite), 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, violet-blue, yellowish-gray, brownish-yellow, or colorless. Strong pleochroism is a key identifier, showing different colors (e.g., deep violet-blue, pale blue, yellowish-gray) when rotated.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to greasy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often occurs as disseminated grains, anhedral to subhedral crystals, or columnar aggregates within metamorphic rocks. Gem-quality material is typically transparent to translucent.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Orthorhombic system. Crystals are typically prismatic, often with pseudohexagonal cross-sections due to twinning. Twinning is common, often cyclic or lamellar.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Distinct to good cleavage on {010}, poor on {100} and {001}. Fracture is conchoidal to uneven.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (schists, gneisses, hornfels) derived from aluminous sediments. Also found in some granites and pegmatites.

Key Facts

Hardness

7-7.5 (Mohs scale)

Specific Gravity

2.53-2.78 g/cm³

Crystal System

Orthorhombic

Color

Blue, violet-blue, yellowish-gray, brownish-yellow, colorless (strongly pleochroic)

Luster

Vitreous to greasy

Transparency

Transparent to translucent

Fracture

Conchoidal to uneven

Cleavage

Distinct to good on {010}, poor on {100} and {001}

Composition

Magnesium iron aluminum cyclosilicate

Physical characteristics

• Crystal Habit: Prismatic, often pseudohexagonal due to twinning; also massive, granular, or columnar aggregates.
• Cleavage Type: Distinct to good in one direction ({010}), poor in two others ({100}, {001}).
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to greasy.

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