DetailsChalk is a soft, white, porous sedimentary carbonate rock, a form of limestone. Cretaceous in age. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint nodules embedded in chalk. Brilliant white chalk cliffs are a dominant feature of the Yorkshire coast & the south coast of England. Specimens are supplied in labelled card trays.