Dark volcanic rock (8)
'dark volcanic rock' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for obsidian that I've seen before include "Dark glassy rock formed from lava" , "A glassy volcanic rock" , "A hard black volcanic glass" , "Glassy black igneous rock" , "Dark volcanic glass" .)
'dark volcanic rock' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for basalt that I've seen before include "Dark fine-grained volcanic rock" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Common volcanic rock" , "Dark rock of volcanic origin" , "rock from 2 [VOLCANO]" .)