Stone: garnet I fancy (7)
I believe the answer is:
granite
'stone' is the definition.
(I know that granite is a type of batholite)
'garnet i fancy' is the wordplay.
'fancy' indicates an anagram.
'garnet'+'i'='garneti'
'garneti' with letters rearranged gives 'GRANITE'.
(Other definitions for granite that I've seen before include "Resilience; stone" , "Tangier (anag.)" , "Crystalline rock" , "Hard igneous rock" , "Tearing (anag.)" .)