Antique French Cameo Necklace

My grandmother passed this piece of jewlery down to me and she owned it for many years. it is a heavy piece, with unmarked gold that has very intricate designs. I'm not sure what the carved center is made up of, but the back of the piece has holes for the material to breathe. 

i have come across many different necklaces with this design listed anywhere from $130-$5000, however I can not find this specific piece of jewlery anywhere. I would love to know more about it!