I just did an experiment to test adamantine vs copper. I put up dwarfs of the exact same skill levels (lvl 4 or Competent for all skills), individually pitting full adamantine armor dwarfs with copper weapons against dwarfs with adamantine weapons and copper armor; I also pitted dwarfs with adamantine weapons and adamantine armor against copper weapons and copper armor. Only dwarfs with the same weapon type were pitted against each other, so the effectiveness of the weapon type was a controlled variable. I only used male dwarfs, no other sex or race, so the effectiveness of the wielder is also a controlled variable. I forgot to include multiple instances of each pair, so I do recommend someone repeat this experiment on a larger scale to confirm.
Results:
A armor A spear vs C armor C spear proved inconclusive since both lost their weapons. Neither one died, but CC lost all his teeth.
A armor A sword vs C armor C spear resulted in AA victory without injury.
A armor A axe vs C armor C axe resulted in CC victory without injury.
A armor C spear vs C armor A spear resulted in AC victory without injury.
A armor C sword vs C armor A sword resulted in CA victory without injury.
A armor C axe vs C armor A axe resulted in CA victory with missing teeth and slight injury to the left arm.
I think we can conclude that adamantine is a poor metal for spears, and that it is best for swords. Try a more heavy metal.