Chalk

Chalk is an extremely soft sedimentary rock that forms under the sea due to the gradual accumulation of plates of calcite (a mineral form of calcium carbonate) and very small amounts of clay and silt. The famous white cliffs of Dover are an extraordinary example of a chalk rock formation, from which these little chunks of rock were gathered. 

Although now replaced by another mineral form of calcium called gypsum, chalk is often associated with its use at the material for drawing on black boards. When pressed against rough surfaces, the mineral readily crumbles leaving a clear mark on the surface. A less commonly known use of chalk is as an ingredient in some toothpaste due to its mild abrasive properties. The smaller piece of chalk has eroded into a beach pebble that has then been bored by bivalves. The distinctive pattern left by these burrowing creatures has then been colonised by seaweeds that have attached themselves to the pebble.