Answer: obsidian

AnswerCrossword Clue
OBSIDIANDark volcanic glass
OBSIDIANVolcanic glass
OBSIDIANIgneous rock
OBSIDIANArrowhead material
OBSIDIANGlassy volcanic rock.
OBSIDIANRelative of perlite
obsidianvolcanic glass similar in composition to granite, usually dark but transparent in thin pieces, and having a good conchoidal fracture
obsidiana volcanic glass