Grease pencils are made from non-toxic wax and can be used to mark or write on all kinds of nonporous surfaces. They are mainly used in commercial and industrial applications and are usually made from non-toxic waxes like paraffin, beeswax, ceresin, carnauba or spermaceti. They’re like strong crayons that you never have to sharpen. All you have to do is just pull the string and peel back the paper to expose more of the wax. Safe to use on surfaces like glass, porcelain, rock, polished stone, plastic and even ceramics. It’s waterproof and will not fade in the sunlight and even easy to erase. Just grab a damp cloth and rub it away.