Diamonds - rocks (3)
I believe the answer is:
ice
This clue is a double definition.
'diamonds' is the first definition.
(slang term for diamonds)
'rocks' is the second definition.
(both can informally mean diamonds)
(Other definitions for ice that I've seen before include "Decorate a cake" , "Cubes in a drink" , "Cool down" , "Do cake decoration" , "Formality" .)