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Prince, Prince, Prince — just the ultimate type of rock (5)

Ross

I think it could be either:

jewel

prase

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

'rock' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for prase that I've seen is " quartz".)

I've seen this clue in The Times and The Sunday Times.
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