Necklace gem (5)
I believe the answer is:
pearl
'necklace gem' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pearl that I've seen before include "Gem from the sea" , "Precious jewel from the oyster" , "Girl < , pretty" , "coming out of shell?" , "Paragon" .)