So we think it could still possibly be an engagement ring? And maybe she was engaged to my Great-Grandfather when this photo was taken (1915-1917).
We know they had children together between 1918-1934. Perhaps they couldn't afford to get married until they did in 1929?
The fact that she uses two different maiden names is still very strange.
I thought I was on to something Treetotal, when I found that Dorothy! Birth nearly matched, place is very close and the fact that she was with an aunt. I've traced that Dorothy to the USA though. She emigrates there and marries there, so not my Nanny Dinah unfortunately

Thanks for the thought though.
And thanks for the link to that forum Sylvia!