Okay, saw a great/fascinating book on Cameo's today...."Cameos - Classical to Costume" 2nd Edition by Monica Lynn Clements and Patricia Rosser Clements published by Schiffer Books.
I didn't know that cameos were/are made from all these: coral, lava, Bakelite, amethyst, hematite, ivory, jade, celluloid, glass, shell, jasper ware (Wedgewood), shell and plastic.
The only ones in the book with necklaces were made from shell...and had nice values. I knew shell, ivory and jade were most common/valuable from way back...
So...you need to determine your base! Hope this helps!