A mineralogical term for obsidian, a dark volcanic glass. The kanji literally mean 'horseshoe stone', possibly from its appearance or historical use.
Bateiseki is a type of volcanic glass, also called kokuyouseki (obsidian).
The kanji 馬蹄 (horseshoe) + 石 (stone) suggest a shape or historical association, but the exact derivation is uncertain. The term is a technical mineral name.