Chalk was a soft, white form of calcium carbonate, often used for drawing or writing on hard surfaces.
Human scientist Robert H. Goddard used a chalk board to note down his rocketry calculations in 1926. (Star Trek: Enterprise opening credits)
In 1953, Kay Eaton rebuffed Julius Eaton's suggestion that Benny Russell print his own stories privately, saying more people would read them if he wrote them in chalk on the sidewalk. A moment later, Benny seemed to agree with making his story about Deep Space 9 a dream, saying getting it published that way would be better than chalk on the sidewalk. (DS9: "Far Beyond the Stars")
In 2367, a hologram of Albert Einstein wrote on a blackboard with chalk as Reginald Barclay and Geordi La Forge looked on. (TNG: "The Nth Degree")
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