Chalk is a pretty common type of rock – probably one of the more commonly encountered. Yup – the every day household ‘chalk’ was originally made of regular chalk. These days, other sources of material are sometimes used – such as gypsum.
Chalk is a form of Limestone, essentially formed by layers and layers of microorganisms shedding their shells over time. Fine examples of Chalk cliffs can be found around the UK – notably the white cliffs of Dover and the Hunstanton chalk formation.