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Black and Gold Obsidian is a naturally occurring volcanic glass characterized by its predominantly black color with distinctive golden, shimmering reflections or 'sheen' when viewed under direct light. This optical phenomenon, known as aventurescence or schiller, is due to the presence of microscopic inclusions within the glass matrix. It is an amorphous material, meaning it lacks a crystalline structure.
How to Identify
- Color
- Predominantly black, with a metallic golden sheen or shimmer visible when light reflects off its surface. The sheen can range from subtle to very pronounced.
- Luster
- Vitreous (glassy) to sub-vitreous.
- Texture
- Smooth, conchoidal fracture surfaces. Non-porous.
- Crystal Form
- Amorphous; lacks crystal structure.
- Cleavage
- None. Exhibits conchoidal fracture.
- Geological Environment
- Found in areas of recent volcanic activity where felsic lava flows have cooled rapidly, typically on the margins of rhyolitic lava flows or domes.
Key Facts
- Hardness: 5-5.5 on Mohs scale
- Specific Gravity: 2.35-2.60
- Crystal System: Amorphous (lacks crystal structure)
- Color: Black with golden sheen
- Luster: Vitreous (glassy)
- Transparency: Opaque to translucent in thin sections
- Fracture: Conchoidal (shell-like, very sharp edges)
- Cleavage: None
- Composition: Primarily Silicon Dioxide (SiO2), typically 70-75% SiO2, with minor amounts of Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, and H2O.
Quick Check
- Color: Black with golden reflections
- Luster: Glassy
- Streak: White (due to its amorphous nature and high silica content, it produces a white streak when powdered, though it's rarely practical to test)
Physical Characteristics
- Crystal Habit: Massive, amorphous
- Cleavage Type: None
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous (glassy)
Formation
Formed by the rapid cooling of felsic lava (rich in silica) with minimal crystal growth. The 'gold' sheen is caused by minute inclusions of gas bubbles or mineral crystals (e.g., hematite, goethite) aligned along flow layers within the glass, which reflect light.
Usage
Historically used for cutting tools, weapons, and mirrors. Modern uses include jewelry, ornamental objects, and as a surgical scalpel material due to its extremely sharp edge. Also used in some industrial applications as an abrasive.
Age Distribution
Typically Cenozoic, but can form whenever rapid cooling of felsic lava occurs.
Where to Find
Mexico
Significant deposits, particularly in areas with extensive rhyolitic volcanism.
United States (Oregon, California, Arizona)
Volcanic regions in the western U.S. are known for various types of obsidian, including sheeny varieties.
Armenia
Known for various obsidian types, including those with optical effects.
Greece (Milos Island)
Historically important source of obsidian, some with sheeny characteristics.
Finding Tips
Look in Volcanic Regions
Focus your search in areas with recent (geologically speaking) rhyolitic volcanic activity, especially near lava flows, domes, and volcanic vents.
Examine Outcrops and Stream Beds
Obsidian can be found in situ within volcanic rock formations or as eroded fragments in stream beds and alluvial deposits downstream from volcanic sources.
Check for Conchoidal Fracture
Obsidian's characteristic conchoidal (shell-like) fracture is a key identifier. Look for sharp, curved edges.
Observe Sheen in Direct Light
The golden sheen is best observed by rotating the specimen under a direct light source (sunlight or a flashlight) to catch the reflective inclusions.
Safety Precautions
Obsidian fragments can be extremely sharp. Handle with care, and consider wearing gloves to prevent cuts. Always obtain permission before collecting on private land or in protected areas.
Similar Rocks
Silver Sheen Obsidian
Obsidian (Volcanic Glass)
Also known as: Silver Obsidian
Rainbow Obsidian
Obsidian (Volcanic Glass)
Also known as: Iridescent Obsidian
Mahogany Obsidian
Obsidian (Volcanic Glass)
Also known as: Red Obsidian
Scientific Classification
- Mineral Class
- Mineraloid (not a true mineral due to amorphous structure)
- Group
- Volcanic Glass
- Crystal System
- Amorphous
- Chemical Formula
- SiO2 (amorphous)
- Composition
- Felsic volcanic glass, predominantly silica (SiO2) with minor impurities.
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