Dark volcanic rock (6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dark volcanic rock</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dark volcanic rock</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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]" .)