Cameo Brooch Set in 10K Gold

To be honest, I know almost nothing about this pendant. It was my great aunt's, but she died when I was a baby. My mom thought it was just a piece of costume jewelry and was going to sell it in a yard sale. My aunt was born in Philadelphia in the early 1900s/10s. She never travelled, but both of her parents were from Italy.
It is stamped 10K on the clasp and there is a maker's mark on the bottom that is very hard to see. I think the mark had 6-7 letters and it looks like an M in the middle. I've looked at the portrait using a loupe and the edges don't look smooth and there are also dark specks in the white that you can't see with the naked eye. The back is concave and the front is definitely not flat, as you can hopefully see in the pictures.
I like to think it is a shell cameo, but I'm not sure, that's why I'm asking for your help. Thanks!