I think those are what they call 'stirrup' or 'horseshoe' rings. They're designed like that. If I'm right, there's a straight bar (a seperate piece) that passes through a loop or hole on one arm of the horseshoe, then through the nipple piercing, then the loop or hole in the other arm. To keep it in place, little caps are snapped or screwed onto each end of the straight piece, like the cap that screws onto a tie-tack.

Anyone else have a guess?