Beading Term: Split Rings
Split rings look just like key rings, only much smaller. Like jump rings, split rings can be used to connect parts of a jewelry piece together, but because of their tight spiral design, they're much more secure, making them a good choice to hang charms from.
Also because of their spiral design, split rings can be tough to separate. That's where split ring pliers come in. One end of the nose of the pliers has a notch for separating the bands of the split ring. I've had some success opening split rings just by sliding my thumbnail between bands of the ring, but that doesn't always work.