Dark green mineral Newton discovered in rocky pine trees (10)
I believe the answer is:
serpentine
'dark green mineral' is the definition.
(I've seen this before)
'newton discovered in rocky pine trees' is the wordplay.
'newton' becomes 'N' (synonyms).
'discovered in' is an insertion indicator.
'rocky' indicates an anagram.
'pine'+'trees'='pinetrees'
'pinetrees' is an anagram of 'serpentie'.
'n' inserted within 'serpentie' is 'SERPENTINE'.
(Other definitions for serpentine that I've seen before include "Winding, twisting; mineral" , "water created in London" , "Snake-like; mineral" , "London water feature" , "London lake" .)