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Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth. It is typically black, but can also be brown, green, or rarely blue or red, depending on impurities. Its characteristic conchoidal fracture produces extremely sharp edges, which have been utilized by humans for millennia.
How to Identify
- Color
- Typically jet black, but can also be dark brown, dark green, or rarely red, blue, or iridescent (rainbow obsidian). The color is due to trace elements and inclusions.
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Smooth, homogeneous, and glassy. May contain small phenocrysts (microlites) or gas bubbles, but generally lacks a crystalline texture.
- Crystal Form
- Amorphous (non-crystalline).
- Cleavage
- None. Exhibits characteristic conchoidal fracture.
- Geological Environment
- Found in areas of recent volcanic activity where felsic lavas (rhyolite, dacite, andesite) have cooled rapidly. Common in lava flows, domes, and margins of dikes and sills.
Key Facts
- Hardness: 5-6 on the Mohs scale
- Specific Gravity: 2.35-2.60
- Crystal System: Amorphous (lacks a crystal structure)
- Color: Black, dark brown, dark green; rarely red, blue, or iridescent
- Luster: Vitreous (glassy)
- Transparency: Translucent to opaque, thin edges may be transparent
- Fracture: Conchoidal (shell-like), producing extremely sharp edges
- Cleavage: None
- Composition: Primarily silicon dioxide (SiO2), typically 70-75% SiO2, with minor amounts of aluminum, sodium, potassium, iron, and magnesium oxides. It is essentially a supercooled liquid, not a true mineral.
Quick Check
- Color: Black, dark brown, dark green (rarely red, blue, or iridescent)
- Luster: Vitreous (glassy)
- Streak: White (due to its amorphous nature and high silica content)
Physical Characteristics
- Crystal Habit: Amorphous masses, flow bands often visible
- Cleavage Type: None
- Fracture Type: Conchoidal, very sharp
- Tenacity: Brittle
- Luster Type: Vitreous (glassy)
Formation
Obsidian forms from the rapid cooling of felsic lava (rich in silica, typically rhyolitic composition) with minimal crystal growth. This rapid quenching prevents the formation of a crystalline structure, resulting in a natural glass.
Usage
Historically, obsidian was highly valued for its sharp edges, making it ideal for cutting tools, arrowheads, spear points, and surgical blades. Modern uses include jewelry, ornamental objects, and as a raw material for some industrial applications (e.g., perlite, though perlite is hydrated obsidian). Its extremely sharp edges have also led to its use in specialized surgical scalpels.
Age Distribution
Typically Cenozoic to recent, as it is prone to devitrification over geological time. Most obsidian is less than 20 million years old.
Where to Find
United States
Significant deposits are found in the western United States, particularly in Oregon (Newberry Volcano, Glass Buttes), California (Glass Mountain, Mono-Inyo Craters), Idaho (Big Southern Butte), Arizona, New Mexico, and Wyoming (Yellowstone National Park).
Mexico
Extensive deposits, historically important for Mesoamerican cultures, are found in central Mexico (e.g., Pachuca, Teotihuacan sources).
Iceland
Common in areas of active volcanism, such as Hrafntinnuhraun.
Italy
Found on volcanic islands like Lipari and Sardinia.
Greece
Melos island is a historically significant source.
Turkey
Anatolia has numerous obsidian sources, important since the Neolithic period.
Japan
Hokkaido and other volcanic regions.
Kenya
Rift Valley region.
Ecuador
Andean volcanic regions.
Finding Tips
Look in Volcanic Regions
Obsidian is exclusively found in areas with relatively recent (geologically speaking) volcanic activity, particularly where rhyolitic or dacitic lavas have erupted.
Identify Lava Flows and Domes
Search for obsidian in the margins of lava flows, volcanic domes, and areas where volcanic glass has been eroded and transported by water.
Check for Conchoidal Fracture
A key identifier is its characteristic conchoidal (shell-like) fracture, which produces sharp, curved surfaces. This is often visible on broken pieces.
Observe Luster
Its distinct glassy luster is usually easy to spot. Wetting the surface can enhance the luster and make it more apparent.
Safety Precautions
Obsidian breaks with extremely sharp edges, comparable to surgical scalpels. Handle with extreme care to avoid cuts. Wear sturdy gloves and eye protection when collecting or breaking samples. Be aware of potential hazards in volcanic terrains, such as unstable ground and sharp rocks.
Similar Rocks
Basaltic Glass (Tachylite)
Tachylite
Also known as: Tachylite
Pitchstone
Pitchstone
Also known as: Hydrated Volcanic Glass
Pumice
Pumice
Also known as: Volcanic Foam
Scientific Classification
- Mineral Class
- Not a true mineral (it is a rock, specifically a volcanic glass)
- Group
- Volcanic Glass
- Crystal System
- Amorphous
- Chemical Formula
- Variable, but predominantly SiO2 (typically 70-75%)
- Composition
- Felsic (rhyolitic) volcanic glass, composed mainly of silica, with minor amounts of Al2O3, Na2O, K2O, FeO, Fe2O3, MgO, CaO, and H2O.
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