Gesso was once alive. Chalk is the main substance in gesso – it is a soft, white, pure form of limestone (calcium carbonate) with a very fine grain. Combined with white pigment (whiting) and rabbit skin glue – its an excellent absorbent prime coat for paintings. Chalk is made up of tiny skeletons or fossils called coccoliths – microscopic plates that were once phytoplankton – an algae living in the ocean millions of years ago.