What is a Blackboard chalk made of?

Geological chalk is a soft limestone made of the plated skeletons of microscopic algae that settled on Cretaceous sea floors, and blackboard chalk was originally exactly that. Modern sticks are usually gypsum-based instead, because casting plaster gives a consistent hardness and a stick that writes without gouging the board. The distinction matters for anyone tempted to try classroom chalk in a garden or an aquarium, where the two minerals behave completely differently. Sidewalk chalk is the same gypsum in a fatter stick, and the "chalk" that gymnasts and climbers use is neither — it is magnesium carbonate, chosen because it absorbs sweat rather than because it makes a mark.

Component by component

Other stationery and paper objects

• Pencil Graphite mixed with clay and fired, glued inside two grooved halves of softwood. No lead, ever.
• Paper Cellulose fibres from wood pulp, plus up to a third mineral filler, sizing and brightener.
• Ballpoint pen A tungsten carbide ball a millimetre across, rolling in a brass socket, fed by thick oil-based ink.
• Glitter Aluminium-coated polyester film, colour-coated and chopped into fragments — by definition a microplastic.
• Crayon Paraffin wax with pigment stirred in, poured hot into a mould and cooled. Almost nothing else.
• Cardboard Two flat kraft paper liners glued either side of a fluted paper medium — the flutes do all the work.
• Eraser Either natural rubber with fine abrasive and oil, or plasticised PVC — two different ways of lifting graphite.
• Playing card Two sheets of paper laminated around a black glue core, then coated in plastic — or solid PVC.
• Postage stamp Coated paper with phosphor bands, printed in security inks, on either water-activated gum or a self-adhesive.