At heart of stone when cut light's an intrinsic quality they say (11)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

<st> O <ne> (“heart of” means middle letter only) + PAL <e> (=light, of colour; “when cut” means last letter is dropped) + homophone (“they say”) of “essence=an intrinsic quality”; semi- & lit. , I suppose