you can try "Złote Tarasy" in downtown there are alot of places there, if you plan to buy diamond ring try to read this education blog [bluenile.com/diamonds/diamond-education] at least you will have basic knowledge, Congratulations.
You could also check shops that offer jewellry designed by young Polish designers:
zero925.pl/strona/2/english.html
i-techne.pl/teksty.php?t=onas
pieknerzeczy.org/index_en.htm
You may find something more original and unique than in chain stores (like Kruk, Apart etc.)
Good luck!