Does anyone know what a "fart rock" is?

I remember as a kid during recess, we'd hunt for "fart rocks" -- they were dark pumice looking black rocks that when crushed would smell like a sulphurous fart. While most of the time we'd find fart rocks that could fit in your hand, I remember one of my friends pulling up this motherlode of a fart rock the size of a hubcap. Up until that point of my life I truly didn't know what it meant to be envious of someone. Does anyone know what these rocks really were?