No heart of stone (7)

I believe the answer is:

diamond

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

'stone' is the definition.
(I know that diamond is a class)

I cannot understand how the remainder of the clue works.

(Other definitions for diamond that I've seen before include "Gem stone" , "Form of carbon" , "DOG" , "Precious stone of pure crystallized carbon" , "Carbon gemstone" .)

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

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