Whitish jewel (4)
I believe the answer is:
opal
'whitish jewel' 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 "Milky-white gemstone with flashes of colour" , "Quartz-like gemstone" , "Opaque gem" , "Iridescent gemstone" , "Iridescent quartz-like stone" .)