What is Chalk ?
Chalk is (noun) 1. a type of soft white rock made of calcium The white cliffs of Dover are formed of chalk. as different as chalk and cheese completely different (informal.) 2. a stick of a white or coloured used for writing on a blackboard, e.g. in a classroom He wrote the dates on the board in coloured chalk.(verb) to mark or write with chalk She chalked the menu for the day on the blackboard.
source: Easier English, Student Dictionary Upper Intermediate Level