EState Shell Necklace no Hallmarks?

by Tony
(Eastbourne East Sussex UK)

This necklace was passed on to my wife from her Father.

Family name Shell.
It would appear to be gold apart from the clasp. I am waiting for a test on the purity of the gold.
There appears to be no hallmark stamp anywhere.
The case and the necklace follow the Shell theme and were thought to be passed down in the family as far back as 1920.
The case has the details of Aboud and Collingwood 46 Conduit Street. (the shop now empty).
I would appreciate any guidance you may be able to offer on how I should view this for insurance purposes or sale.