Graduate’s key identifying igneous rock (6)
I believe the answer is:
basalt
'igneous rock' is the definition.
(I know that basalt is a type of volcanic rock)
'graduate's key' is the wordplay.
'graduate's' becomes 'BA's' (someone with a Bachelor of Arts degree).
'key' becomes 'alt' (the Alt keyboard button).
'bas'+'alt'='BASALT'
'identifying' is the link.
(Other definitions for basalt that I've seen before include "Common volcanic rock" , "Type of rock is a blast, oddly" , "Dark rock of volcanic origin" , "Black volcanic rock" , "rock from 2 [VOLCANO]" .)