The ruby ring is summoned by high level mages. Mages start summoning these rings, but the first they get is jade, then opal and finally the ruby. I have a 50 wiz and she loves to go out with these bracelets, especially when soloing.
These are actually bracelets, not rings. We do summon focus effect rings, but those grant spell casting haste.
What I don't like is that the opal bracer works on spells only up to level 52, yet we don't get an upgrade until lvl 54. It's not a huge deal, but once one gets used to that damage bonus one doesn't like to be without it!