Circles unevenly scratched in rocks (3)
I believe the answer is:
ice
'scratched in rocks' is the definition.
'ice' can be an answer for 'rocks' (both can informally mean diamonds). I am not certain of the 'scratched in' bit.
'circles unevenly' is the wordplay.
'unevenly' means one should take alternating letters (letters in odd-numbered positions rather than even).
The alternate letters of 'circles' are 'ICE'.
Can you help me to learn more?
(Other definitions for ice that I've seen before include "Being feeble, can't cut any" , "Decorate with frosting" , "Frozen water; kill (slang)" , "Cool down" , "Glaciate" .)