Pumice stones are extremely useful for removing calluses and cleaning stains from stubborn spots. However, a pumice stone won't last forever. Knowing when to replace it is important in order to maintain healthy skin and good hygiene.
For Household Use
Some pumice stones are used to get stains off materials like porcelain, and can be used to get rid of fabric pills. These stones are usually block shaped, and fairly large. A good sign that the stone needs to be replaced is when it becomes small and too smooth to be effective.
For Personal Use
Other pumice stones are designed to be used on human skin. These are the stones that get rid of dry spots or calluses on feet and heels. These stones should also be replaced when they're too small and too smooth to use effectively.
Breaking and Crumbling
Pumice stones break off or start to crumble during use. This doesn't necessarily mean they need replacing. In fact, that simply means they're doing their job. And remember, it's better to replace a stone that has worn away than to add extra pressure while using it. Like dull knives, you could accidentally injure yourself; rubbing off too much skin because of pressure is not something to be desired.