How to identify Mahogany Obsidian

To correctly identify Mahogany Obsidian (Volcanic Glass (SiO2 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 brown to black base with reddish-brown (mahogany) patches, streaks, or swirls. The red is due to iron oxide inclusions.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, non-porous, and amorphous. When broken, it exhibits a conchoidal (shell-like) fracture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; no crystal form.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its amorphous nature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas of recent volcanic activity where felsic lavas have cooled rapidly, typically on the margins of lava flows, domes, or in volcanic ash beds.

Key Facts

Hardness

5 - 5.5 on the Mohs scale

Specific Gravity

2.35 - 2.60

Crystal System

Amorphous (lacks a crystal structure)

Color

Black to dark brown with reddish-brown streaks/patches

Luster

Vitreous (glassy)

Transparency

Opaque to translucent on thin edges

Fracture

Conchoidal (shell-like), producing very sharp edges

Cleavage

None

Composition

Primarily SiO2 (silica) with minor amounts of Al2O3, Fe2O3, FeO, MgO, CaO, Na2O, K2O, and H2O. The reddish color is due to iron oxide (hematite/goethite) inclusions.

Physical characteristics

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

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