basalt

Concise Oxford English Dictionary © 2008 Oxford University Press:
basalt /ˈbasɔːlt, -(ə)lt/

▶noun a dark fine-grained volcanic rock composed largely of plagioclase with pyroxene and olivine.

– derivatives
basaltic /bəˈsɔːltɪk/ adjective.

– origin C17: from L. basaltes (var. of basanites), from Gk basanitēs, from basanos ‘touchstone’.