These objects are not made of the same material.
The walls of Minas Tirith are only stated to be similar to Orthanc:
For the main wall of the City was of great height and marvellous thickness, built ere the power and craft of Númenor waned in exile; and its outward face was like to the Tower of Orthanc, hard and dark and smooth, unconquerable by steel or fire, unbreakable except by some convulsion that would rend the very earth on which it stood.
(Return of the King, Siege of Gondor, my emphasis.)
The Palantiri did not come from Numenor; they were made in the First Age by the Noldor in Valinor:
The palantiri came from beyond Westernesse from Eldamar. The Noldor made them. Feanor himself, maybe, wrought them, in days so long ago that the time cannot be measured in years.
(Two Towers, The Palantir)
Gurthang was only noted to have been blackened after the death of Beleg; before that (when it was named Anglachel) there is no evidence that it was black (despite what the wikis might say):
And Gwindor gave the sword Anglachel into his hands, and Túrin knew that it was heavy and strong and had great power; but its blade was black and dull and its edges blunt.
(The Silmarillion, Of Túrin Turambar)
Furthermore, Anglachel cannot be said to have any particular efficacy against the enemy on account of having come from Arda Unmarred (if that is indeed the case, since Arda represents the Solar system, not the Earth); it is in fact particularly noted by Melian to be anything but "unmarred":
There is malice in this sword. The dark heart of the smith still dwells in it.
(The Silmarillion, Of Túrin Turambar)
There are therefore no inferences to be drawn from the material that these objects are made of, and you're reading connections into this that just aren't there in the source texts.
