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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 dark in color, though its color can vary depending on impurities. Its characteristic conchoidal fracture makes it easily identifiable and historically significant.
How to Identify
- Color
- Typically dark, ranging from black to dark brown or dark green. Reddish or brownish hues can occur due to hematite inclusions. 'Snowflake obsidian' contains spherulites of cristobalite, giving it a mottled appearance.
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Smooth, homogeneous, and glassy. Absence of visible crystals to the naked eye.
- Crystal Form
- Amorphous (lacks a crystalline structure).
- Cleavage
- None. Exhibits characteristic conchoidal fracture.
- Geological Environment
- Found in areas of recent volcanic activity where felsic lavas have cooled rapidly, such as lava flows, domes, and margins of dikes and sills.
Key Facts
- Hardness: 5-5.5 on the Mohs scale
- Specific Gravity: 2.35-2.60 g/cm³
- Crystal System: Amorphous (non-crystalline)
- Color: Black, dark brown, dark green; can have iridescent sheens (rainbow, gold, silver) due to microscopic inclusions or gas bubbles.
- Luster: Vitreous (glassy)
- Transparency: Opaque to translucent in thin sections or at edges.
- Fracture: Conchoidal (shell-like, producing very sharp edges).
- Cleavage: None.
- Composition: Primarily SiO₂ (typically 70-75%), with minor amounts of Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O, and H₂O (up to 1%).
Quick Check
- Color: Black, dark brown, dark green (rarely red/gold sheen)
- Luster: Vitreous (glassy)
- Streak: White (due to its amorphous nature and high silica content)
Physical Characteristics
- Crystal Habit: Massive, amorphous.
- Cleavage Type: None.
- Fracture Type: Conchoidal, producing extremely sharp edges.
- 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 an amorphous glass.
Usage
Historically, obsidian was highly valued for tool-making (e.g., arrowheads, knives, scrapers) due to its conchoidal fracture producing extremely sharp edges. Modern uses include surgical scalpels (due to its superior sharpness compared to steel), jewelry, decorative objects, and as a geological specimen.
Age Distribution
Typically Cenozoic to recent, as it devitrifies over geological time.
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 areas like Pachuca, Hidalgo, and near Teotihuacan.
Ecuador
Volcanic regions in the Andes contain obsidian deposits.
Japan
Known for its obsidian deposits, particularly on Hokkaido and Honshu islands.
Turkey
Anatolia has historically significant obsidian sources, used since the Neolithic period.
Greece
The island of Milos is a famous ancient source of obsidian.
Italy
Sardinia and Lipari islands are known for obsidian.
Iceland
Volcanic regions contain obsidian, often associated with rhyolitic eruptions.
New Zealand
North Island volcanic zones have obsidian occurrences.
Finding Tips
Look for Volcanic Regions
Obsidian is exclusively found in areas with recent (geologically speaking) felsic volcanic activity. Focus your search in regions known for rhyolitic lava flows or domes.
Examine Outcrops and Stream Beds
Look for dark, glassy fragments in weathered outcrops, scree slopes, or stream beds downstream from volcanic sources. Its distinctive conchoidal fracture is a key indicator.
Check for Associated Minerals
Obsidian often occurs with other volcanic rocks like rhyolite, pumice, and tuffs. The presence of these can indicate a potential obsidian source.
Safety Precautions
Obsidian edges can be extremely sharp, comparable to surgical scalpels. Handle specimens with care, preferably wearing thick gloves, to avoid cuts. Always obtain permission before collecting on private land or in protected areas.
Similar Rocks
Basaltic Glass
Tachylite
Also known as: Tachylite
Pitchstone
Pitchstone
Also known as: Hydrated Volcanic Glass
Chert
Chert
Also known as: Flint
Scientific Classification
- Mineral Class
- Not a true mineral (amorphous, not crystalline)
- Group
- Volcanic Glass
- Crystal System
- Amorphous
- Chemical Formula
- Variable, primarily SiO₂ (approx. 70-75%)
- Composition
- Silica-rich (felsic) volcanic glass, typically rhyolitic in composition. Contains minor amounts of various oxides (Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O) and dissolved water.
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