the gremlins must have it in for me .... just when I go to post I get that little line of red wording staring at me......... apologies for seeming to double up on Christine's notes, but I'll let it stand.................
strongly suspect you're on to a loser as to provenance/attribution - don't know when this might have been made, but the shape and wavy rim look as though they might owe more to arts and crafts/nouveau styles than later periods, although bulges in the middle do occur in some deco shapes. Of course this might have been made much later, and simply be another example of copying.
Always tempting to think that a piece of glass had its origins in the country in which you have found it - but this might have been from almost anywhere - although not unreasonable to narrow it down to Europe
Your diamonds look as though they might be cross-cut, but the picture is too small, and loses details when blown up, so can't be sure. You are allowed a first dimension of 700, so you can see how much small this one is.
You don't mention wear - if this were really early C20 then the cutting would be very sharp and there should be substantial wear on the foot, and I think it would have been lead glass. You're correct with your description of the depression under the foot.
Cut glass does suffer from damage and wear - perhaps more so than other types - and something like this if it did have a big age under its belt would be no exception. If it's pristine then probably modern.
Cut glass has been made in prodigious quantities for a long time - much of it unmarked - and collecting the stuff comes always with problems like this, plus the added disadvantage of a lack of good books and it seems few collectors.