I personally do not currently wear any jewellery but only as i've not found anything i've liked. I do however see many people with rings cufflinks etc.
One item i have seen used and i really really want but haven't yet found is worn by a lodge brother and is a silver (with s&C logo) napkin hook. 1 end hooks into the tie knot the other is a small clamp that holds the napkin. at the FB we were all joking about it and subsequently all threw food down our ties and the item was immediately added to the wish list.