Dark fine-grained volcanic rock (6)
I believe the answer is:
basalt
'dark fine-grained 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 rock of volcanic origin" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Type of rock is a blast, oddly" , "Common volcanic rock" , "Black volcanic rock" .)