chalk is a white soft material
I remember having to use that instead of dry erase markers.
chalk is a form of limestone
I had no idea, I thought limestone was totally different.
Chalk
Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores. Flint (a type of chert) is very common as bands parallel to the bedding or as nodules embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled
to facilitate soldering, removing rust, and many other applications. As an example of an unusual use, a fibreglass eraser was used for preparing an archaeological fossil embedded in a very hard and massive limestone.
Types Other.
Felt chalkboard erasers or blackboard dusters are used to erase chalk markings on a chalkboard. Chalk writing leaves light-coloured particles weakly adhering to a dark surface (e.g., white on black, or yellow on green); it can be rubbed off with a soft material, such as a rag.