Chalk It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are then consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacture now may utilize mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to
Blackboard bold from the attempt to write bold letters on blackboards in a way that clearly differentiated them from non-bold letters i.e. by using the edge rather than point of the chalk. It then made its way back into print form as a separate style from ordinary bold, possibly starting with the original 1965 edition of Gunning and Rossi's textbook on complex analysis. Some mathematicians do not recognize blackboard bold as a separate style from bold. Jean-Pierre Serre uses double-struck letters when writing bold on the blackboard, whereas his published works consistently use ordinary bold for the same symbols. Donald Knuth also