Value Of My Antique Cameo Enquiry

by Sarah
(Somerset, UK)

Please could you give me a rough value for this Cameo. I'm not 100% sure of its age but I know that it first came into my family when my great great great grandmother had it.
The are two marks on the back, one is a diamond shape with N M written in it and I think the other mark says 9CT.

As I said, I don't know much but I would say it's in excellent condition