Garnet confused with one type of stone (7)
I believe the answer is:
granite
'type of stone' is the definition.
'garnet confused with one' is the wordplay.
'confused with' indicates an anagram.
'one' becomes 'i' (Roman numeral).
'garnet' anagrammed with 'i' makes 'GRANITE'.
(Other definitions for granite that I've seen before include "Crystalline rock" , "Get rain (anag) - rock" , "A very hard rock from Tangier" , "determination" , "Hard stone from Tangier" .)