Currently there is no emoji that could explictly represent the concept of 'metal' that I know of.
A bar / ingot of metal (grey) would be nice.

The following:🪙,⚙️, and perhaps🤘

Could maybe currently be used to represent the concept in a more abstract sense but an ingot / bar of metal would probably also have semantic uses outside of the just 'metal' (heaviness, value, resillience etc.)