An Auld Wee Locket frae me Gran and Gramps and Scotland.

This is a locket that I've always been fascinated with since I was a child. It was always at my grandparent's home, stuffed in with the photo albums and loose photos, that they kept in a cabinet. If they ever told me who she was, I have long since forgotten.
I'm hoping that someone with some knowledge of the style of dress or hairstyle could shed a little light on a date range. I'm thinking she's in mourning, but then, who was the locket for? Maybe she just liked wearing black.
The picture itself is metal, though I'm not sure what type, and there was never any chain, that I can remember. There was nothing behind the photo or scratched on the back that I could find.
I also did not see any silversmith names, sterling or purity numbers anywhere in or on the locket.
It's very pretty though, and there is still paint on the beautiful Scottish Thistle. It's in great shape, I'm excited for any information anyone could give me about anything on it!