[petascale]

Quoting the article:

"While a few other substances (like solid neon) could potentially explain the coma-free acceleration, hydrogen was the best match for the data."

It doesn't have to be hydrogen. But to explain acceleration from solar heating it does need outgassing, and the outgassing has to be in a form that we can't detect (no comet tail). Stone won't have much (or any) of that.