These are small rare earth magnets - 3/16" rings. Composed of Neodymium/Iron/Boron (NdFeB), these magnets are curiously strong and make for a great way to attach tools to a metal workbench, hold small children to a fridge, and as a trigger for reed switches. This one small ring can easily hold a piece of paper to your fridge.
We found these small rings to be very useful for connecting PCBs together. These rings are conductive and can act as a magnetic connector between widgets. You need a way to secure the ring to the PCB (screws and rivets have been shown to work).
Sold in single units.