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Mahogany Obsidian is a naturally occurring volcanic glass characterized by its dark, typically black or very dark brown base color, interspersed with reddish-brown to mahogany-colored patches or swirls. These reddish hues are caused by the presence of iron oxide inclusions, such as hematite or goethite, which are dispersed within the silica-rich glass matrix. It is an amorphous material, meaning it lacks a defined crystalline structure. Like all obsidian, it exhibits a conchoidal fracture, producing very sharp edges when broken.
How to Identify
- Color
- Dark brown to black base with reddish-brown (mahogany) patches, streaks, or swirls. The red is due to iron oxide inclusions.
- Luster
- Vitreous (glassy).
- Texture
- Smooth, non-porous, and amorphous. When broken, it exhibits a conchoidal (shell-like) fracture.
- Crystal Form
- Amorphous; no crystal form.
- Cleavage
- None, due to its amorphous nature.
- Geological Environment
- 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.
Quick Check
- Color: Dark brown to black with reddish-brown patches
- Luster: Vitreous (glassy)
- Streak: White (though typically not performed on glass due to hardness)
Physical Characteristics
- Crystal Habit: Massive, amorphous
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous
Formation
Mahogany Obsidian forms from the rapid cooling of felsic lava (rich in silica) with a low water content. The rapid quenching prevents the formation of a crystalline structure, resulting in a natural glass. The distinctive reddish-brown patches are due to inclusions of iron-rich minerals, primarily hematite or goethite, which oxidize during or after the cooling process.
Usage
Historically, obsidian was used for tools, weapons (arrowheads, spear points), and cutting implements due to its conchoidal fracture producing extremely sharp edges. Today, Mahogany Obsidian is primarily used in lapidary arts for cabochons, beads, carvings, and ornamental objects. It is also popular in metaphysical and spiritual practices.
Age Distribution
Typically Cenozoic to recent, as it is an unstable glass that devitrifies over geological time.
Where to Find
United States
Significant deposits are found in Oregon (e.g., Glass Buttes), California, Arizona, New Mexico, and Utah.
Mexico
Various volcanic regions throughout Mexico yield Mahogany Obsidian.
Armenia
Known for its historical and contemporary obsidian sources, including varieties similar to Mahogany Obsidian.
Japan
Volcanic regions produce various types of obsidian.
Iceland
Active volcanic areas are sources of obsidian.
Finding Tips
Look in Volcanic Areas
Search near recent (geologically speaking) volcanic flows, domes, and ash deposits. Obsidian often occurs as nodules, flow bands, or larger masses.
Identify Glassy Luster
Obsidian has a distinct glassy sheen. Look for dark, smooth, shiny rocks that break with sharp, curved edges.
Check for Reddish Inclusions
Specifically for Mahogany Obsidian, look for the characteristic reddish-brown streaks or patches within the dark glass. This distinguishes it from plain black obsidian.
Safety Precautions
Obsidian breaks with extremely sharp edges. Handle with care, and consider wearing gloves and eye protection when collecting or breaking samples. Avoid inhaling obsidian dust if cutting or grinding, as it contains fine silica particles which can cause silicosis.
Similar Rocks
Black Obsidian
Obsidian
Also known as: Volcanic Glass
Snowflake Obsidian
Obsidian (with cristobalite inclusions)
Also known as: Flowering Obsidian
Apache Tears
Obsidian
Also known as: Obsidian Nodules
Scientific Classification
- Mineral Class
- Mineraloid (natural glass, not a true mineral due to lack of crystalline structure)
- Group
- Volcanic Glass
- Crystal System
- Amorphous
- Chemical Formula
- SiO2 (amorphous) with trace elements and iron oxide inclusions
- Composition
- Silica-rich (typically 70-75% SiO2) with varying amounts of other oxides (Al, Fe, Mg, Ca, Na, K) and water. Iron oxide inclusions (hematite, goethite) give it its characteristic mahogany color.
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