How to identify Mahogany Obsidian

To correctly identify Mahogany Obsidian (Volcanic Glass (Obsidian with iron inclusions)), 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 hues are 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, glassy, and amorphous. Can feel sharp along fracture edges.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; lacks crystal structure. 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 silicic volcanic activity, typically associated with rhyolitic lava flows or domes.

Key Facts

Hardness

5 to 5.5 on the Mohs scale (due to its amorphous structure, it's softer than quartz but still hard enough to scratch glass).

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 on thin edges.

Fracture

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

Cleavage

None.

Composition

Primarily silicon dioxide (SiO₂) with minor amounts of aluminum, sodium, potassium, iron, and other trace elements. The reddish-brown color is due to iron oxide (hematite or goethite) inclusions.

Physical characteristics

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

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