Stone in large masses (3)

I believe the answer is:

gem

'stone' is the definition.
(a gemstone)

'in large masses' is the wordplay.
'in' says the answer is hidden in the clue.
'GEM' can be found hidden inside 'large masses'.

(Other definitions for gem that I've seen before include "Valued person; jewel" , "Half of 7 [SIGN OF THE ZODIAC]" , "Emerald or sapphire, say" , "one of the Cairngorms?" , "Treasured person" .)

I've seen this clue in the Evening Standard.

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