Are you sure about that? According to Wikipedia both gypsum and calcite are used. Apparently, gypsum is used for colored chalk, and calcite is used for white chalk:
> Chalk sticks are produced in white and in various colours, especially for use with blackboards. White chalk sticks are made mainly from calcium carbonate derived from mineral chalk or limestone, while coloured chalk sticks are made from calcium sulphate in its dihydrate form, CaSO4·2H2O, derived from gypsum.[6][7] Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO3 (calcite).
https://en.wikipedia.org/wiki/Blackboard#Chalk_sticks
Wikipedia cites the following articles:
[6] "How chalk is made – material, making, used, processing, procedure, product, industry". madehow.com. Retrieved February 17, 2021.
[7] Corazza, M.; Zauli, S.; Pagnoni, A.; Virgili, A. (2012). "Allergic contact dermatitis caused by metals in blackboard chalk: a case report". Acta Dermato-Venereologica. 92 (4): 436–437. doi:10.2340/00015555-1296. PMID 22367154.
The first of these seems more relevant... I'm not quite sure what the second citation adds.
> Chalk sticks are produced in white and in various colours, especially for use with blackboards. White chalk sticks are made mainly from calcium carbonate derived from mineral chalk or limestone, while coloured chalk sticks are made from calcium sulphate in its dihydrate form, CaSO4·2H2O, derived from gypsum.[6][7] Chalk sticks containing calcium carbonate typically contain 40–60% of CaCO3 (calcite).
https://en.wikipedia.org/wiki/Blackboard#Chalk_sticks
Wikipedia cites the following articles:
[6] "How chalk is made – material, making, used, processing, procedure, product, industry". madehow.com. Retrieved February 17, 2021.
[7] Corazza, M.; Zauli, S.; Pagnoni, A.; Virgili, A. (2012). "Allergic contact dermatitis caused by metals in blackboard chalk: a case report". Acta Dermato-Venereologica. 92 (4): 436–437. doi:10.2340/00015555-1296. PMID 22367154.
The first of these seems more relevant... I'm not quite sure what the second citation adds.