Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Mahogany Obsidian is a naturally occurring volcanic glass characterized by its dark, glassy texture and distinctive reddish-brown to black banding or blotches. These patterns are caused by the presence of iron-rich mineral inclusions, which differentiate it from common black obsidian. It is an amorphous material, meaning it lacks a crystalline structure, and exhibits a characteristic conchoidal fracture.
How to Identify
- Color
- Dark reddish-brown to black, often with swirling patterns or blotches of these colors. The red-brown hues are due to iron oxide inclusions.
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Smooth, glassy, and amorphous. Can feel sharp along fracture edges.
- Crystal Form
- Amorphous; lacks crystal structure. Occurs as massive flows, nodules, or fragments.
- Cleavage
- None. Exhibits conchoidal fracture.
- Geological Environment
- 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.
Quick Check
- Color: Dark reddish-brown to black, often mottled or banded.
- Luster: Vitreous (glassy).
- Streak: White (due to its amorphous nature and high silica content, though often difficult to obtain a streak due to hardness and fracture).
Physical Characteristics
- Crystal Habit: Massive, amorphous.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy).
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 to black coloration and swirling patterns 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 as an ornamental stone, for lapidary work (cabochons, spheres, carvings), and in jewelry. It is also popular in metaphysical and spiritual practices.
Age Distribution
Typically Cenozoic, but can form whenever silicic volcanism occurs.
Where to Find
Mexico
Significant deposits are found in various volcanic regions of Mexico, particularly in areas with rhyolitic volcanism.
United States (Oregon, Arizona, California)
Abundant in the western United States, especially in states known for extensive volcanic fields such as Oregon (e.g., Glass Buttes), Arizona, and California.
Armenia
Known for various types of obsidian, including those with iron inclusions.
Japan
Volcanic regions in Japan also yield obsidian, including varieties with reddish hues.
Finding Tips
Geological Context
Look for Mahogany Obsidian in areas with recent (Cenozoic) silicic volcanic activity, particularly near rhyolite flows, domes, or volcanic vents. It often occurs as surface float or within weathered volcanic deposits.
Appearance
Identify its characteristic glassy luster, dark color with reddish-brown streaks or patches, and sharp, conchoidal fracture. Freshly broken surfaces will be very sharp.
Safety Precautions
Obsidian breaks with extremely sharp edges, comparable to surgical scalpels. Always wear thick gloves and eye protection when handling or breaking obsidian. Be cautious of sharp fragments. While not inherently toxic, the fine dust created during cutting or grinding can be an inhalation hazard (silica dust), so proper ventilation and respiratory protection (e.g., N95 mask) are recommended for lapidary work.
Similar Rocks
Black Obsidian
Obsidian
Also known as: Volcanic Glass
Snowflake Obsidian
Obsidian with cristobalite inclusions
Also known as: Flowering Obsidian
Apache Tears
Obsidian nodules
Also known as: Marekanite
Scientific Classification
- Mineral Class
- Not a true mineral (amorphous), but classified as a mineraloid.
- Group
- Volcanic Glass
- Crystal System
- Amorphous
- Chemical Formula
- SiO₂ (with impurities and trace elements)
- Composition
- Felsic volcanic glass, typically 70-75% SiO₂, with iron oxides (e.g., Fe₂O₃) responsible for the mahogany coloration.
Explore Mahogany Obsidian
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.