Volcanic glass (8)
I believe the answer is:
obsidian
'volcanic glass' is the definition.
(obsidian is a kind of volcanic glass)
This is all the clue.
(Other definitions for obsidian that I've seen before include "Dark volcanic glass" , "Dark volcanic rock" , "A glassy volcanic rock" , "Glass bottle may be in this" , "Glassy black igneous rock" .)