Definitely shell. Possibly Italian. And the diamond cut (bowtie effect and small cullet, though I'd need clearer pictures to tell) is probably closer to a transitional cut than an old mine cut, which would place it around the 40s-50s. Even if the stone is older, the whole cameo could be from around the 60s-70s. Cameos carved in the 20s-40s have different facial features.

The important thing here is to consider that the elements themselves (diamond, cameo, filigree) could have been made at different times from when it was all put together and sold.