What are the charms called u put on bolt ring necklaces- they usually have a clasp so they don't fall off?

i have a thick bolt ring necklace that i want to get a charm for- i previously thought they were called an enhancer, however these simply slide through the clasp as you do it up or un do it. i thought you could get one that actually clips onto the link in-between the bold joins. can anyone help me with what they are called?