No heart of stone (7)
I believe the answer is:
diamond
'stone' is the definition.
(I know that diamond is a class)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for diamond that I've seen before include "Gem stone" , "Form of carbon" , "DOG" , "Precious stone of pure crystallized carbon" , "Carbon gemstone" .)