How to identify Mahogany Obsidian

To correctly identify Mahogany Obsidian (Volcanic glass (silica-rich)), 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

   Dark reddish-brown to black, often with swirling patterns or blotches of these colors. The red-brown is due to iron oxide 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, conchoidal fracture surfaces. Amorphous, non-crystalline.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; lacks crystal form. Occurs as massive flows, nodules, or fragments.

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 or geologically young volcanic activity, typically associated with rhyolitic lava flows, domes, and ignimbrites.

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

Dark reddish-brown to black, often with swirling patterns.

Luster

Vitreous (glassy).

Transparency

Opaque to translucent in thin pieces.

Fracture

Conchoidal (shell-like, very sharp edges).

Cleavage

None.

Composition

Primarily silicon dioxide (SiO₂) with minor amounts of iron oxides (hematite, goethite) responsible for the color, and other trace elements.

Physical characteristics

• Crystal Habit: Massive, amorphous, typically occurring as flows, nodules, or fragments.
• Cleavage Type: None.
• Fracture Type: Conchoidal, producing extremely sharp edges.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy).

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