辛 Xīn — the Yin Metal Day Master

What the classics read it as

Yin Metal. The jewel — jade already cut, gold already worked. The value is in the finishing, and the finishing is already done.

「辛金軟弱，溫潤而清」. Xin metal is soft, warm to the touch, and clear. It is the only metal in the system described with the vocabulary of comfort, and the whole of its character sits in that contrast: hard material, gentle surface, and no tolerance at all for being reworked.

What it loves: 「樂水之盈」 — it delights in water at the full. Water washes it, and a worked stone that is washed shows what it is; a worked stone under dust is indistinguishable from an unworked one. 「熱則喜母，寒則喜丁」 — in heat it wants its source, in cold it wants 丁, the lamp. Not 丙. The sun overwhelms this stem, and the small managed flame warms it exactly.

What it fears: 「畏土之疊」 — it dreads earth in layers. Being buried. In this system earth is support, and the specific failure of 辛 is not attack but accumulation: too much backing, too much material, and the finished thing is lost inside it.

The character: precise, and easily insulted by force. Correction lands well and pressure does not. 「能扶社稷，能救生靈」 says the classical line — it can hold up a state and it can save the living — which is a startling claim for the smallest metal and is meant to be.

The cost: it is not rinsed clean. It is polished, by someone, over time, and it does not survive being hurried.