This is a Shell Cameo in a fine filigree 14K surround. It is unusual because it is in a rectangular surround instead of the typical oval.
It is 1 1/4" by 1" and weighs 6.1 grams.
It has a secure clasp hallmarked 14K and has a bale so that it can be worn as a pendant. The subject has lovely curls and flowers on her gown. There is a yellow tinge to the cameo which is no doubt a remnant from the shell used. Just different enough to be noticed.
14K Filigree Cameo Pin/Pendant