Pumice is a product of vulcanism. The rock is full of gas, and it can float on water. When ground up, it makes an excellent abrasive for sanding. It's been used for polishing furniture and, if I'm not mistaken, teeth. (Your dentist may have a problem with that.) But it's also great for making pastel surfaces. When mixed with gesso and applied to a substrate, it becomes the "grit" that holds the pastel.
I wonder how far my 2 lb bag will go? It looks like an awful lot of pumice to me. And it would be easy to confuse it with stone-ground wheat flour.