How to identify Obsidian

To correctly identify Obsidian (Volcanic glass), 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 dark, ranging from black to dark brown or dark green. Reddish or brownish hues can occur due to hematite inclusions. 'Snowflake obsidian' contains spherulites of cristobalite, giving it a mottled appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to sub-vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, homogeneous, and glassy. Absence of visible crystals to the naked eye.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (lacks a crystalline structure).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Exhibits characteristic conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas of recent volcanic activity where felsic lavas have cooled rapidly, such as lava flows, domes, and margins of dikes and sills.

Key Facts

Hardness

5-5.5 on the Mohs scale

Specific Gravity

2.35-2.60 g/cm³

Crystal System

Amorphous (non-crystalline)

Color

Black, dark brown, dark green; can have iridescent sheens (rainbow, gold, silver) due to microscopic inclusions or gas bubbles.

Luster

Vitreous (glassy)

Transparency

Opaque to translucent in thin sections or at edges.

Fracture

Conchoidal (shell-like, producing very sharp edges).

Cleavage

None.

Composition

Primarily SiO₂ (typically 70-75%), with minor amounts of Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O, and H₂O (up to 1%).

Physical characteristics

• Crystal Habit: Massive, amorphous.
• Cleavage Type: None.
• Fracture Type: Conchoidal, producing extremely sharp edges.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy).

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