Whitish gemstone (4)
I believe the answer is:
opal
'whitish gemstone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opal that I've seen before include "item in Hatton Garden?" , "what may be set in 29 [DIAL]" , "Many-coloured gem" , "Jewellery item" , "Semi-precious stone of b;uish white colour" .)