What kind of stone was it? In Oaxaca Mexico, there is caldo de piedra (literally stone soup) which involves heating up a riverstone really hot and dropping it into a bowl of your ingredients to get it to immediately boil

Of course you have to know which type of stone to use as many can explode when put into a fire.

These stones are also carefully selected and passed down through generations.

Maybe it was actually one of these? https://en.wikipedia.org/wiki/Stone_Soup