Item Chests Help

Okay.

Is Luck a factor in this game? Cuz I don't see it in my stats so I assume it isn't, but then is there a way to increase my chances of getting the better items out of item chests and jars?

I know about the Diamond Armlet thingy and I didn't think chains had anything to do with getting better things from anything but monsters.

My problem is that I always get like 23 gil from a jar that also holds say Shielded Armor. hmmm.... that makes me wonder... How long does it take for jars to reappear?