Which article should be used in this sentence? [closed]

I have to fill the blank with an article (a,an or the)

Copper is _ useful metal.

Metal is a noun and it is qualified by an adjective 'useful'. But I can't find any rule pertaining to this situation and hence I'm not sure which article would be suitable.

Please provide me the answer with an appropriate reason.

Short answer: a should be used because copper is one one example under the category of useful metals which also could include aluminum, iron, gold, silver, among others.

In isolation, that is the answer most speakers would choose

Long Answer: It depends on the circumstance. To give one example:

An engineer is talking to their apprentice through why copper is (surprisingly to the apprentice) the metal used for a specific application

You would think steel would be used here, right? Most people do but this situation is different. Copper is the useful metal

This is a scenario where what is considered "useful" has been more narrowly defined, to where copper is the only member of the group (or at least the most useful of the members.) Therefore it is the metal as opposed to a metal.