Obsidian
Obsidian is not a mineral but a glass, formed where lava cools too fast to crystallise. It fractures conchoidally, which means it can be knapped to an edge finer than surgical steel, and Mesoamerican and Mediterranean toolmakers exploited that for millennia.
Polished slabs served as mirrors long before silvered glass. The sixteenth-century English figure John Dee owned an Aztec obsidian mirror that now sits in the British Museum, and its provenance did as much for the stone's later reputation as any folklore.
The dark, reflective, edged qualities all converge in later traditions that treat it as a stone of boundaries and honest reflection.