Fireplace feature inlaid with nickel and stone (7)
I believe the answer is:
granite
'stone' is the definition.
(I know that granite is a type of batholite)
'fireplace feature inlaid with nickel' is the wordplay.
'fireplace feature' becomes 'grate' (I've seen this in another clue).
'inlaid with' is an insertion indicator.
'nickel' becomes 'Ni' (Ni is the chemical symbol for nickel).
'grate' placed around 'ni' is 'GRANITE'.
'and' is the link.
(Other definitions for granite that I've seen before include "Hard granular rock" , "Coarse-grained igneous rock" , "Showing great resolution" , "Get rain (anag) - rock" , "determination" .)