Autumn birthstones (5)
I believe the answer is:
opals
'autumn birthstones' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for opals that I've seen before include "Precious stones" , "Milky sparkling gemstones" , "Semi-precious stones of milky or bluish colour" , "jewels" , "Translucent gemstones" .)