Ceramic(Hard ferrite) magnets were developed in the early 1960 as a low-cost alternative to metallic magnets (steels and alnico), Ceramic magnets are very hard and brittle. Standard tolerances for ceramic magnet dimensions are +/-.005" for ground dimensions and +/- 2% of the size for as sintered dimensions. Because of their brittleness these magnets will not withstand impact or flexing. Ceramic magnets are chemically inert non-conductors, which is a benefit in many applications.                     http://www.sensmagnetics.com/