Gain monopoly in rocks important in building (12)

I believe the answer is:

cornerstones

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'gain monopoly in rocks important in building' is the definition.
I can't tell whether this defines the answer.

'gain monopoly in rocks' is the wordplay.
'gain monopoly in' becomes 'corner' (I can't explain this - if you can you should give a lot more credence to this answer).
'rocks' becomes 'stones' ('stone' can be a synonym of 'rock').
'corner'+'stones'='CORNERSTONES'

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I've seen this clue in The Globe and Mail.

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