Crossword Genius

Rock, not slime, converted energy (9)

Ross

I believe the answer is:

limestone

'rock' is the definition.
(I know that limestone is a type of rock)

'not slime converted energy' is the wordplay.
'converted' indicates anagramming the letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'not'+'slime'='notslime'
'notslime' with letters rearranged gives 'limeston'.
'limeston'+'e'='LIMESTONE'

(Other definitions for limestone that I've seen before include "building material" , "Hard calcium rock" , "Tie lemons up in sedimentary rock" , "Clint" , "Basis of cement" .)

I've seen this clue in The Daily Express.
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