Metal is one of the five elements of Wǔ Xíng. The ancient meaning of 金 (now obsolete) was “copper”. Ponte Ryūrui says that 金 “is a pictograph of copper ore cast in a mould”. You are unlikely to remember that, however, unless you have studied metallurgy. Alternatively, you can think of 金 as a representation of a Chinese bell made of bronze or some other copper alloy. Uncle Hanzi gives a number of historical forms of this character that look convincingly like such bells.