I'm in a spot where if I asked about their ring, it'd be mostly just out of honest curiosity, maybe as a conversation point; not trying to hit on the girl; I might just think it's a cool design and want to store the info away for later.
I'm not quite sure what the difference is between a Promise ring, a Purity ring, a Wedding ring, or an Engagement ring, because a lot of hem are starting to look very similar.
I also know several girls who wear rings in order to ward off the creepy guys (they're single), or they do it to fit in to certain cultures (work, for example) and I know several people who don't wear any rings (and have been married for years). I knew a guy who would wear a ring to get women to flirt with him and boost his confidence (it was also for a character). Though, if I look, and I see a ring on that hand, I'm going to naturally assume she's in a serious relationship, and would not mess with that.