A chalkboard or blackboard is a reusable writing

Writing

Writing is the representation of language in a textual medium through the use of a set of signs or symbols . It is distinguished from illustration, such as cave drawing and painting, and non-symbolic preservation of language via non-textual media, such as magnetic tape audio.Writing most likely...

 surface on which text or drawings are made with sticks of calcium sulfate

Calcium sulfate

Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...

 or calcium carbonate

Calcium carbonate

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

, known, when used for this purpose, as chalk

Sidewalk chalk

Sidewalk chalk are typically large colored sticks of calcium sulfate mostly used for drawing on pavement or cement sidewalk. It is sometimes used to draw a four square court or a hopscotch board.-Use:...

. Chalkboards were originally made of smooth, thin sheets of black or dark grey slate

Slate

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

 stone. Modern versions are often green because the colour is considered easier on the eyes.
A chalkboard can simply be a piece of board painted with matte dark paint

Paint

Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...

 (usually black or dark green).