The rock is rock or sand that has been soaked in silver fulminate. Silver fulminate (like mercury fulminate, which would be toxic) is explosive. However, the quantity of fulminate in Pop Its is very small so the little exploding rocks are safe. Fulminates are easily prepared by reacting metal with concentrated nitric acid. You don't want to go making this in any quantity yourself because the fulminate is shock sensitive and pressure sensitive. However, if you decide to make do-it-yourself Pop Its, the silver fulminate is more stable if flour or starch is added to the crystals during the filtering process.