Diamonds — really cool stuff (3)
I believe the answer is:
ice
This clue might be a double definition.
'diamonds' is the first definition.
(slang term for diamonds)
'cool stuff' is the second definition.
(ice is cool or cold)
'really' acts as a link.
I am not very happy about this link. It may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for ice that I've seen before include "Frozen liquid" , "If it's thin, skating on it can be dangerous" , "Decorate a cake" , "Solid water" , "Diamonds, colloquially" .)