How to identify Obsidian

To correctly identify Obsidian (Obsidian), 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 jet black, but can also be dark brown, dark green, or rarely red, blue, or iridescent (rainbow obsidian). The color is due to trace elements and inclusions.

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. May contain small phenocrysts (microlites) or gas bubbles, but generally lacks a crystalline texture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (non-crystalline).

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 (rhyolite, dacite, andesite) have cooled rapidly. Common in lava flows, domes, and margins of dikes and sills.

Key Facts

Hardness

5-6 on the Mohs scale

Specific Gravity

2.35-2.60

Crystal System

Amorphous (lacks a crystal structure)

Color

Black, dark brown, dark green; rarely red, blue, or iridescent

Luster

Vitreous (glassy)

Transparency

Translucent to opaque, thin edges may be transparent

Fracture

Conchoidal (shell-like), producing extremely sharp edges

Cleavage

None

Composition

Primarily silicon dioxide (SiO2), typically 70-75% SiO2, with minor amounts of aluminum, sodium, potassium, iron, and magnesium oxides. It is essentially a supercooled liquid, not a true mineral.

Physical characteristics

• Crystal Habit: Amorphous masses, flow bands often visible
• Cleavage Type: None
• Fracture Type: Conchoidal, very sharp
• Tenacity: Brittle
• Luster Type: Vitreous (glassy)

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