i know its some kind of reactant but what?
The secret ingredient is silver fulminate, according to this Wikipedia article:
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Snap rocks are gravel impregnated with a minute quantity of silver fulminate (AgCNO). Silver fulminate is a percussive that detonates under impact. Snap rocks are considered safe because the mass of the percussive is miniscule in comparison to that of the mass of the gravel.