igneous
Obsidian
Volcanic Glass (Amorphous Silicon Dioxide)
Hardness of 5 to 5.5 on the Mohs scale, typically jet black or dark brown, vitreous (glassy) luster, amorphous crystal structure with conchoidal fracture, and specific gravity around 2.35 to 2.45.
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Physical properties
Hardness of 5 to 5.5 on the Mohs scale, typically jet black or dark brown, vitreous (glassy) luster, amorphous crystal structure with conchoidal fracture, and specific gravity around 2.35 to 2.45.
Formation & geological history
Formed as an extrusive igneous rock when felsic lava cools rapidly with minimal crystal growth, typically originating from rhyolitic eruptions.
Uses & applications
Used historically for sharp cutting tools, arrowheads, and blades, as well as in modern jewelry, decorative carvings, and occasionally in surgical scalpel blades.
Geological facts
Despite looking like modern glass, obsidian is a naturally occurring rock. Its extremely sharp edges can be finer than surgical steel blades.
Field identification & locations
Easily identified by its shiny, glassy appearance, conchoidal fracture pattern, and lack of visible crystal grains. Found worldwide in areas with past volcanic activity.
