Durak, I was going to suggest you bring your jewellery gift into Ukraine, when after reading your other post it seems you are already there! (But bear in mind for future that the Ukrainian Customs Officials seem to be obsessed with foreigners importing jewellery, so check current regulations to avoid their imposition of any taxes. (Although some might suggest a chain is the easiest thing to bring in tax-free: just wear it!)
A good-value jewellery shop anywhere in the world will be difficult to find with recent massive increases in all the commodity prices, but even so there will always be some shops better than others. Forget all those retailers in the fashionable central shopping areas, because their high shop-rental expenses are reflected in their stock prices. I would start by going to the much cheaper 'Metro-grad' (large underground arcade-tunnel of small retailers) - anyone will know where this is. It's quite fun to see anyway and it's not touristy. It has a large Internet cafe at one far end; so you can report your progress! Apparently there is another arcade 'Globus' at Maydan Nezalezhnosti, but I have never been there.
If your mind is made up then you may not need my opinion, but I should advise that almost all NEW jewellery you see in shops is worth its melt (scrap) price only. (Not applicable to most antique variety.) And the smaller 'precious' stones generally have no value. (Just for information, Friday's gold scrap buying price [8.4.'11] was ~ $27 per gram for 14ct, so you definitely won't buy refined 14ct gold cheaper than ~ $33 per gram. All that new poor quality stuff in the 'High Street' shops (sorry, but it mostly is) you can buy at SCRAP PRICE, if you know where to go. As for the gem stone, I would recommend a good quality semi-precious stone of GOOD COLOUR, in preference to a small sorry-looking precious stone. Jewellery taste is such a personal thing that I would either let her choose, or at least ask her what metals, stones, colours, sizes and types of it that she likes and dislikes. I have given wonderful pieces to friends and family that I know they never wear. So, it's cheap chocolates or flowers for them next time!
Perhaps, but I have been noticing an increasing number of signs (especially in the metro) that have both Cyrillic and Latin letters in Saint Petersburg, as well. Perhaps it is in recognition of the economic value of western tourism.