A rock you may have in your hand (7)

I believe the answer is:

diamond

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'a rock' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for diamond that I've seen before include "DOG" , "Ace" , "Suit-baseball field - gem" , "Sixtieth" , "Kohinoor, Hope" .)

I've seen this clue in the Evening Standard.

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