igneous
Likely black obsidian
Obsidian (natural volcanic glass; typically rhyolitic, silica-rich composition)
AI-generated identification · may be incorrectBlack, strongly vitreous luster with curved, conchoidal fracture and very fine texture; no visible crystals or cleavage. Mohs hardness about 5–5.5; specific gravity approximately 2.35–2.65. The glossy triangular fragment is consistent with a freshly fractured piece.
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Physical properties
Black, strongly vitreous luster with curved, conchoidal fracture and very fine texture; no visible crystals or cleavage. Mohs hardness about 5–5.5; specific gravity approximately 2.35–2.65. The glossy triangular fragment is consistent with a freshly fractured piece.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot grow, producing volcanic glass. It is generally Quaternary to Cenozoic in age and occurs around rhyolitic lava flows and volcanic domes.
Uses & applications
Used historically for extremely sharp cutting tools and arrowheads; today it is collected, carved, polished, and occasionally used in jewelry or decorative objects. It has little value as ordinary rough material.
Geological facts
Obsidian edges can be sharper than surgical steel because fracture occurs at a microscopic scale. Black coloration commonly results from iron and other trace elements, though brown, gray, and banded varieties also occur.
Field identification & locations
Identify by glassy luster, lack of grains, curved fracture surfaces, and sharp edges; a hardness or streak test can help, but avoid scratching the specimen. Common locations include Mexico, the western United States, Armenia, Turkey, and Japan.
