AIBU to ask for help identifying this diamond ring?

[FelicityFlockheart]

Any experts about? I have this ring and I can't for the life of me remember where it came from. It was possibly my great grandmas and maybe one before that.

I can't even tell what metal it's set in, don't think it's yellow gold. It is set with 15 diamonds. It's hallmarked but with what?

Any ideas? See photos!

Another pic

I can't tell anything from that photo, but this might assist:

theassayoffice.com/anatomy-of-a-hallmark

If they are diamonds then it's likely gold or platinum. The hallmark is rubbed quite a bit but looks like the date letter is an O, and to me the letter style looks like 1890s. Have a look at hallmarks online, see if you can identify the middle one, which will tell you the assay office, and the last one, which will tell you the date. The first mark tells you what the metal is but it looks pretty rubbed away in your pic.