生 (raw / uncooked) contrasts with
熟 (cooked / processed).
生石膏 is the unprocessed mineral CaSO₄·2H₂O;
熟石膏 (plaster of Paris) is the calcined form CaSO₄·½H₂O. The same
生/
熟 distinction shows up across Chinese herbology - e.g.
生地 vs.
熟地 (raw vs. processed rehmannia root).