Diterpene cyclases that can form multiple diterpene products. (2E,6E,10E)-geranylgeranyl diphosphate = cembrene C + diphosphate (2E,6E,10E)-geranylgeranyl diphosphate + H2O = (R)-nephthenol + diphosphate kcat is 2.8 min(-1). Homodimer. Belongs to the terpene synthase family.