Chalk are sea creatures.

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I learned in geology class that the natural chalk used in school are made of fossilized sea creatures, mostly shelled ones rich in calcium. The bond is weak so they erode easily when used on a chalkboard. They don't use chalk for school here in US as much anymore, now they use erasable markers on white board. Markers are very convenient though it has odor but chalk, it makes a powder and makes the room dusty. Chalk is still used in the gym though. I use them when I lift heavy weights.