Metal is the phase of consolidation, the point where the Wu Xing's outward motion turns inward again. In the generative, or Sheng, order Earth feeds Metal, ore drawn from the ground, and Metal in turn feeds Water, as minerals dissolve and mist forms on a cold blade. In the controlling, or Ke, order Metal controls Wood, an axe felling a tree, while Metal is itself controlled by Fire, which melts it. The pairing of feeding and controlling relationships around every phase is what lets the system diagnose an imbalance as a disturbance in a relationship rather than a fault in a single element.
In Chinese medicine Metal governs the Lung and its paired Large Intestine, and by extension the skin, the nose, and the capacity to let go, both of breath and of grief, the emotion the phase is said to carry. Metal's season is autumn, its direction west, and its colour white, correspondences that place it at the year's contraction, when growth is cut back and stored.
Metal's controlling role over Wood makes it, in five-phase diagnosis, the check on unrestrained growth and anger; a weakened Metal phase is read as an inability to set a boundary, whether in breath, in grief, or in temperament, letting Wood expand unchecked.
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