The fine and ultra-fine grit stones are "closed-celled" (the particles that make them up are very dense) and will show little or no wear over time. The medium grit stones are "open-celled" (particles are less dense) and will wear only slightly over ti me. Although ceramics are hard materials - measuring up to 9.22 in hardness on the Mohs Scale - they may still be chipped or broken if not handled with care. Please note that our ceramics are guaranteed against manufacturing defects only.