Powder that's rarely valuable (4,4)
I believe the answer is:
gold dust
I believe this clue is a double definition.
'powder' is the first definition.
(I have seen 'Precious powder' mean 'gold dust' so perhaps 'powder' could also mean 'gold dust')
'that's rarely valuable' is the second definition.
(gold dust can refer to anything valuable and rare)
(Other definitions for gold dust that I've seen before include "Proverbially rare stuff" , "something much sought-after" , "Fine powder of precious metal" , "Something valuable (and very rare?)" .)