Mordecai practically jizzed himself when shown one, so Carl went back and got several hundred.
Carl showed a sheol rock to Mordecai, who reacted strongly enough to tell him to retrieve as many as possible. So on the last visit down the employee line he asks Fire Brandy whether she can part with any, and she lets him take several hundred still-burning pieces along with a few dozen that have not yet caught fire. She will not give him all of it: she needs to keep the fire burning nice and hot to keep her babies happy, for as long as it lasts.
Burning black rocks, and a smaller pile of unlit ones.
An alchemical material Mordecai wants in bulk, sourced from a demon's fire.