Chalk and coal are both sedimentary rocks, but they are formed from different materials and processes:

* Chalk: Formed primarily from the accumulation of the calcium carbonate skeletons of microscopic marine organisms called coccolithophores. This makes chalk a type of biogenic sedimentary rock.

* Coal: Formed from the accumulation and compression of plant matter, often in swampy environments. This makes coal a type of organic sedimentary rock.