Nigari is produced from seawater after first removing sodium chloride. It contains mostly magnesium chloride, smaller amounts of magnesium sulfate (Epsom salt), potassium chloride, calcium chloride, and trace amounts of other naturally occurring salts. 
Nigari was the first coagulant used to make tofu in Japan. It is still used today because tofu made using bittern preserves the original flavor of the soybeans used to make it. Bittern causes rapid coagulation which influences the quality of the tofu. Alternatively calcium sulfate, calcium chloride or other substances are also used.