These discs and rings are made of neodymium, the most powerful magnet material in the world! The largest size disc is only the size of two dimes, but strong enough to hold a .25” thick pad of 3” x 3” sticky notes to a metal filing cabinet! Great for projects that require extreme strength in a small size. These magnets are NOT FOR CHILDREN. Care should be taken because these magnets are brittle and can chip if dropped.