This rare earth magnet for gold testing measures 3/4" in diameter and 3/8" in thickness and features a 3/8" center. Their thickness provides for a strong pulling force of 22 lbs. These rare earth magnets are made of grade N42 neodymium, iron & boron magnetic alloy blend. They also are plated in a nick-copper-nick coating which is the best available. Please note that all neodymium rare earth magnets are fragile and require caution to prevent shattering and to avoid injury to pinching.