igneous
Polished black obsidian
Volcanic glass; predominantly rhyolitic silica glass (approximately 70–80% SiO₂)
AI-generated identification · may be incorrectTypically black, very dark brown, or mahogany with vitreous luster and no visible crystals; Mohs hardness about 5–5.5, conchoidal fracture, no cleavage, and specific gravity around 2.35–2.65. This rounded piece appears polished and may show subtle brown or golden translucence.
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Physical properties
Typically black, very dark brown, or mahogany with vitreous luster and no visible crystals; Mohs hardness about 5–5.5, conchoidal fracture, no cleavage, and specific gravity around 2.35–2.65. This rounded piece appears polished and may show subtle brown or golden translucence.
Formation & geological history
Forms when silica-rich lava cools so rapidly that crystals cannot develop, commonly during Quaternary volcanic eruptions. Major sources include Mexico, the western United States, Armenia, Iceland, and Japan.
Uses & applications
Used for carvings, cabochons, jewelry, ornamental objects, and historically razor-sharp blades and tools. It is primarily a decorative and collecting material rather than a construction stone.
Geological facts
Obsidian is technically a mineraloid rather than a true mineral because it lacks a crystalline structure. Mahogany or sheen varieties contain microscopic inclusions that create colored reflections.
Field identification & locations
Identify it by its glassy appearance, smooth conchoidal fracture, sharp edges when broken, and lack of grains or cleavage; it should not scratch glass reliably as quartz does. The specimen’s exact variety cannot be confirmed from the photograph alone.
