Garnet, I suspect, hard rock (7)
I believe the answer is:
granite
'hard rock' is the definition.
(I've seen this before)
'garnet i suspect' is the wordplay.
'suspect' is an anagram indicator.
'garnet'+'i'='garneti'
'garneti' is an anagram of 'GRANITE'.
(Other definitions for granite that I've seen before include "Constituent in Dartmoor" , "Hard granular rock" , "Showing great resolution" , "determination" , "Crystalline rock" .)