Stone found in remarkably robust planes (9)

'planes' is the definition.
(turbojet is a kind of plane)

'stone found in remarkably robust' is the wordplay.
'stone' becomes 'jet' (I have seen 'Black stone ' mean 'jet' so perhaps 'stone' could also mean 'jet').
'found in' means one lot of letters goes inside another.
'remarkably' indicates an anagram (I've seen this in other clues).
'robust' with letters rearranged gives 'turbos'.
'jet' going into 'turbos' is 'TURBOJETS'.