At first glance, I thought Joe's necklace was a dog tag, but it's a bit too long for that. It also looks like there's something printed on it - maybe it's a printed script necklace like this? I can see that appealing to him.

Interestingly, he's not wearing the necklace in the scene right before he and Nicky get kidnapped, although he is wearing the rings. Although looking at it now, I'm not sure there is more than one ring - it looks like just one really tall one.