Fiery gemstone (4)
I believe the answer is:
opal
'fiery 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 "Quartz-like gemstone" , "Mineral, usually iridescent" , "Gem stone" , "often semi-transparent" , "Glassy gemstone" .)