How to identify Mahogany Obsidian

To correctly identify Mahogany Obsidian (Volcanic Glass (Obsidian) with iron-rich 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 brown to black with reddish-brown (mahogany) patches or swirls. The red 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, glassy, amorphous. May feel sharp along fracture edges.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; lacks crystal structure. Inclusions may be microcrystalline.

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 felsic volcanic activity, typically associated with rhyolitic lava flows, domes, and volcanic ash deposits. It forms where lava cools very rapidly, preventing crystal growth.

Key Facts

Hardness

5 to 5.5 on the Mohs scale (due to its amorphous nature, 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

Black to dark brown with reddish-brown (mahogany) patterns.

Luster

Vitreous (glassy).

Transparency

Translucent to opaque in thicker pieces; thin edges can be translucent.

Fracture

Conchoidal (shell-like, very sharp edges).

Cleavage

None.

Composition

Primarily silicon dioxide (SiO2), typically 70-75%, with minor amounts of aluminum oxide (Al2O3), sodium oxide (Na2O), potassium oxide (K2O), iron oxides (Fe2O3, FeO), and magnesium oxide (MgO). The mahogany color is due to dispersed iron oxide (hematite or goethite) inclusions.

Physical characteristics

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

Explore Mahogany Obsidian