Ball or bead chain connectors. Will work on Chain - 1.8 mm Ball Chain - Matte Black (Foot) - although they seem a little large. The gap is very small on this, and you can get it on the chain, but getting it off is a real struggle. Great if you want it on there permanently. If your intention is to use it as a clasp, try opening the gap a little with round nose pliers before putting on the chain.
3.5mm x 9mm (6)