USGS · Yellowstone obsidian
Cite: U.S. Geological Survey, Yellowstone Volcano Observatory. Tool-making lava flows, natural volcanic glass, conchoidal fracture. https://www.usgs.gov/observatories/yvo/news/yellowstones-tool-making-lava-flows
Mineral
Obsidian · volcanic glass
Obsidian is natural volcanic glass. It cools too fast to crystallize, so it is glass rather than a crystal lattice. People still group it with crystals. Fracture is conchoidal. Cards below cite USGS volcano pages.
Cited public sources
USGS · rhyolite anatomy
Cite: U.S. Geological Survey, Yellowstone Volcano Observatory. Obsidian is volcanic glass that cools so rapidly crystals do not form. https://www.usgs.gov/observatories/yvo/news/rock-glass-and-flowbands-yellowstones-rhyolite-anatomy