Some champagne is suitable for hard rock (6)

I believe the answer is:

gneiss

I'm a little stuck... Click here to teach me more about this clue!

'hard rock' is the definition.
'gneiss' can be an answer for 'rock' (I've seen this before). I am not sure about the 'hard' bit.

'some champagne is suitable' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'GNEISS' is hidden in the letters of 'champagne is suitable'.

'for' acts as a link.

Can you help me to learn more?

(Other definitions for gneiss that I've seen before include "Coarse-grained rock; sounds like agreeable" , "Metamorphic rock" .)

I've seen this clue in The Times.

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