Mineral derived from sago palm (4)

I believe the answer is:

opal

'mineral' is the definition.
(I've seen this before)

'derived from sago palm' is the wordplay.
'derived from' indicates a hidden word.
'OPAL' can be found hidden inside 'sago palm'.

(Other definitions for opal that I've seen before include "Type of gemstone" , "One's stony" , "Semi-precious stone of milky or bluish colour" , "Milky-white mineral" , "Gem-stone with a milky quality" .)

I've seen this clue in The Everyman puzzle in The Observer.

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