Drips/pebbles made from scrap colored porcelain slip dripping from a plaster mold onto paper. I sanded down the surface so they weren’t gritty.
I have been lucky enough to work at a ceramic manufacturing company that uses colored porcelain and plaster molds. Every once and a while I would find these oblong shapes that perfectly formed when dripping off of a plaster mold. They were so fragile. You can still see the ripples they made as the slip pooled on the paper beneath them.