Metal (jin) is one of the Wu Xing, the five phases of Chinese cosmology. It is the phase of contraction, refinement, and cutting away, associated in the classical correspondences with the direction west, the colour white, the season of autumn, the emotion of grief, and the pungent or acrid taste. In the generating cycle (sheng) Metal produces Water, condensation gathering on its cool surface; in the controlling cycle (ke) Metal restrains Wood, the blade cutting the tree. These cycles of mutual generation and control order the relations among the five phases and underlie their application in Chinese medicine, the almanac, feng shui, and the art of fate calculation. These are the conventions of the Chinese cosmological tradition, in which the five phases are bound into a comprehensive system of correspondence across directions, seasons, organs, colours, and feelings.
Metal follows Earth in the generating cycle (Sheng), on the reasoning that ore is drawn from the ground, and Metal in turn generates Water, since traditional metallurgy held that condensation gathers on a cold metal surface. In the controlling cycle (Ke), Metal fells Wood, as an axe cuts a tree, while Fire controls Metal by melting it. The Huangdi Neijing assigns Metal to the lungs and large intestine, the pungent or acrid taste, the colour white, the direction west, and the season of autumn, and associates it with grief and with the process of contraction and refinement, gathering the loose and cutting away the unneeded. In four pillars (BaZi) astrology a chart strong in Metal is read as disciplined and decisive, but liable to rigidity without the softening influence of Fire.
Sources Science and Civilisation in China, Volume 2: History of Scientific ThoughtJoseph Needham with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown, Chinese Mathematical Astrology: Reaching Out to the StarsHo Peng Yoke