Rare minerals affected rock group (8,6)

I believe the answer is:

precious stones

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'rare minerals' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how one could define the other.

'affected rock group' is the wordplay.
'affected' becomes 'precious' (I can't explain this - if you can you should give a lot more credence to this answer).
'rock group' becomes 'Stones' (the Rolling Stones).
'precious'+'stones'='PRECIOUS STONES'

Can you help me to learn more?

I've seen this clue in The Daily Mail.

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