Zi Ran Tong

Zi Ran Tong is not a plant but the mineral iron pyrite (iron disulfide, FeS2), known in English as pyrite or "fool's gold." It occurs as brassy yellow to pale gold cubic or pyritohedral crystals, or as massive granular aggregates, with a metallic lustre and characteristic greenish-black streak. The hardness is 6-6.5 on the Mohs scale and the specific gravity approximately 5.0. For Chinese medicinal use, the mineral is collected, cleaned of matrix rock, and then traditionally calcined and quenched repeatedly in vinegar (cu zhi) to render it more brittle and to enhance extraction. Sourced widely from sulfide ore deposits in China and elsewhere.