Alum blocks are made of natural potassium alum, a material found on rocks in areas of weather and oxidation which has antiseptic and astringent qualities. Alum blocks are used by many for healing minor nicks and cuts from shaving. Another popular use is as an aftershave treatment. It can help fight razor burn, improve skin tone, prevent ache, and kill germs. Alum blocks are typically small, rock-like blocks which look clear to milky. They can last for up to two years, even with daily use.