When an earring is resized, repaired or made into another piece of functioning jewelry this often alters or removes important details on the antique piece. 
A resized piece may have maker’s marks removed or polished off making it difficult to identify exactly who made the piece.
In this picture the jewels in the watch came from an emerald bracelet. The image is from Lang Antiques, they have excellent knowledge library articles on identifying jewelry marks.
Mary Ann Doucette 