Mineral used in plaster of Paris (6)

I believe the answer is:

gypsum

(Other definitions for gypsum that I've seen before include "Plaster of Paris is made from this chalk-like substance" , "White mineral in cement and plaster" , "Mineral used in making plaster of Paris" , "Base of plaster of Paris" , "Spy mug made from this white mineral" .)

I've seen this clue in The Guardian.

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