Mettle refers to what a person is made of.
It is a person’s ability to cope well with difficulties; endurance; determination; courage.

1) France showed their mettle by defeating Argentina in the FIFA World Cup.
2) It’s time for you to prove your mettle
3) The competition will test her mettle.

metal=opaque, lustrous elements that are good conductors of heat and electricity
They are generally malleable, ductile and dense compared to other elements.

1) Aluminium is a metal and hence readily conducts heat.
2) metal detectors
3) Gold is the most precious of all metals