How many kinds of chalk are there and what are they?

There are several types of chalk, primarily categorized into three main kinds: natural chalk, which is a soft, white limestone composed mainly of calcium carbonate; blackboard chalk, typically made from gypsum or calcium carbonate and used for writing on blackboards; and sidewalk chalk, a larger, often brightly colored chalk made from calcium sulfate or other materials for outdoor use. Additionally, there are specialty chalks, such as those used in sports (like gym chalk) and art (like pastel chalk). Each type serves different purposes based on its composition and intended use.