Sold as a paired set of 2.
 
These are high strength neodymium magnets that have been meticulously sealed TWICE for corrosive resistance, water proof, and high impact resistance.
The exterior is ABS plastic.
The interior is an inert compound that's additionally seals the magnet and resists high impact force such as a hammer.
Strength, a single magnet can easily hold a full size hammer on your tool chest.
Two magnets can hold most devices to material 0.5" thick.