At heart of stone, when cut, light's an intrinsic quality, they say (11)
I believe the answer is:
opalescence
'at heart' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Maybe there's an association between them I don't understand?
'of stone when cut light's an intrinsic quality they say' is the wordplay.
'of' becomes 'o''.
'stone when cut light's an' becomes 'pal' (I can't justify this - if you can you should believe this answer much more).
'intrinsic quality' becomes 'essence' (I've seen this before).
'they say' indicates a 'sounds like' (homophone) clue.
'essence' sounds like 'escence'.
'o'+'pal'+'escence'='OPALESCENCE'
Can you help me to learn more?
(Other definitions for opalescence that I've seen before include "Milky iridescence" , "Play of light" , "Shimmering effect" , "Shifting colour" , "Milky iridescent quality" .)