My next option to try would be baking soda and vinegar, but try it on an inconspicuous part of the fireplace first. Sprinkle the baking soda and then use a spray bottle of vinegar to spray the baking soda. I think apple-cider vinegar may work best because I think it is slightly more acidic, but I've used this recipe with white vinegar to clean my garbage disposal. It will bubble up, which should help to get into the pores of the limestone, and then you'll just have to use a wet rag to soak up the excess water. The baking soda will dissolve almost completely, so that shouldn't be an issue. Hope this helps.

Pour hydrogen peroxide on it and scrub with an old toothbrush.

Limestone is porous and vinegar will dissolve the limestone, mixing baking soda with vinegar is pointless. You should never use vinegar on marble or limestone.

This post was edited by moviegeek on Thu, Jan 8, 15 at 16:55

If cornstarch had been used first, it would have absorbed the fudge grease. Sometimes more than one application would be needed to completely remove the stain.

However, as it is, most likely the various liquid cleaning that has been done may have caused the stain to penetrate deeper into the stone. Limestone is very porous.

Contact a stone place and see what they suggest now.