Cameo brooch

I bought this in an op shop and was told by the assistant that an antiques person, who does their valuations on the more expensive and older items, had said the surrounding is 9ct gold, although there is no hallmark.

Am wondering if the straight nose, c-shaped clasp and strange safety pin can narrow down its age for me. And if indeed the surrounding could be 9ct gold. It looks like it to me, rather than rolled yellow gold.

Apart from the cameo being a particularly fine one, it is translucent when held up to light, so this suggests shell, doesn't it?

I am keen to know anything about the jewellery techniques with which it has been constructed/mounted, and its worth. Many thanks.