I think they changed that because if the rings were the same, thats +9 Nat AC (if they stack)
Well then there is no point in putting it in the rings then, is there?
The other major difference is the crafting recipe I believe. An emerald set in mithril amulet for example requires 3 emeralds and 2 ingots of mithril. (Someone confirm this from Raven). I'm assuming the ruby amulets require the same. So it's a bit more involved.