Autumnal birthstone (4)
I believe the answer is:
opal
'autumnal birthstone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opal that I've seen before include "Milky-white gem" , "Quartz-like gemstone" , "One's stony" , "Form of hydrated silica, often used as a gemstone" , "Gem with delicate colours" .)