Chalkboard chalk is calcium sulphate, whereas climbing chalk is magnesium carbonate. You would not want to use chalkboard chalk for climbing, as it would make

Chalkboard chalk is calcium sulphate, whereas climbing chalk is magnesium carbonate. You would not want to use chalkboard chalk for climbing, as it would make holds actually slipperier. Some climbers do carry a piece of chalkboard chalk to mark key holds, though.