Blue gem (8)
I believe the answer is:
sapphire
'blue gem' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sapphire that I've seen before include "Birthstone for September" , "His paper can be made into a jewel" , "Precious thing" , "His paper can be turned into a jewel" , "Precious stone of rich blue" .)