Any birthstone (3)
I believe the answer is:
gem
'any birthstone' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gem that I've seen before include "Pleasant and cheerful (from German)" , "perfect specimen" , "A precious stone like a diamond or emerald etc" , "Emerald or sapphire, say" , "Valuable stone" .)