Powder that is rarely expensive? (4,4)

I believe the answer is:

gold dust

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'powder' is the definition.
(I have seen 'Precious powder' mean 'gold dust' so perhaps 'powder' could also mean 'gold dust')

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for gold dust that I've seen before include "Something valuable (and very rare?)" , "Proverbially rare stuff" , "something much sought-after" , "Fine powder of precious metal" .)

I've seen this clue in The Times.

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