How to identify Schiller Obsidian

To correctly identify Schiller Obsidian (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 (black, dark brown, dark gray) with an iridescent sheen (rainbow, gold, silver) visible when light reflects off internal layers.

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, conchoidal fracture surfaces. Non-porous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; lacks crystal structure. May contain microscopic crystallites (microlites) or gas bubbles that cause the schiller effect.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas of recent felsic volcanic activity, typically associated with rhyolitic lava flows, domes, and ignimbrites. Occurs in the outer, rapidly cooled margins of these volcanic bodies.

Key Facts

Hardness

5 to 5.5 on the Mohs scale.

Specific Gravity

2.35 to 2.60 g/cm³.

Crystal System

Amorphous (lacks a crystal system).

Color

Black, dark brown, dark gray, with iridescent sheens (rainbow, gold, silver).

Luster

Vitreous (glassy).

Transparency

Translucent to opaque, thin edges may be transparent.

Fracture

Conchoidal (shell-like, very sharp edges).

Cleavage

None.

Composition

Primarily silicon dioxide (SiO₂) with minor amounts of aluminum, sodium, potassium, iron, and other trace elements. Typically 70-75% SiO₂.

Physical characteristics

• Crystal Habit: Amorphous mass, lacking external crystal form.
• Cleavage Type: None.
• Fracture Type: Conchoidal, producing extremely sharp edges.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy).

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