We used to live in Bentley in Hampshire. An area of chalk with clay soil and flint houses. Flint is a very hard stone - it's a form of igneous chalk, takes the pressure and time of geology to turn into flint which has a pale exterior with a hard, glassy interior. Was walking across a field near Bentley one day and saw this flint stone lying on the ground. Something about the shape made me pick it up and look and I reckon it's a fossilised sea urchin. Probably lived on the bed of a warm tropical sea many millions of years ago where Bentley is now.