I was watching The Lord of the Rings: The Fellowship of the Ring the other day, and was wondering why the Morgul blade that Aragorn picks up (after it stabbed Frodo) disintegrated when he touched it? In one of The Hobbit movies (I believe the second or third one) Gandalf brings a Morgul blade to Rivendell after exploring the tomb which held the nine, and he is able to carry it without having the blade disintegrate. So I am wondering why the blade disintegrates in one case but not the other.
Credit from original scifi answer.
From the books:
[Aragorn] stooped again and lifted up a long thin knife. There was a cold gleam in it. As Strider raised it they saw that near the end its edge was notched and the point was broken off. But even as he held it up in the growing light, they gazed in astonishment, for the blade seemed to melt, and vanished like a smoke in the air, leaving only the hilt in Strider's hand.
We don't know for sure why it vanished.
This answer on scifi covers the subject well enough. The main theories are that it either disappears after Aragorn touches it (so it might dissolve when a non-Nazgûl touches it), when the light hits it, or because a part of the blade has been broken off.
The answer, however, is only speculative, as Tolkien provided very little info on the subject.
In my opinion, the blade appears to disappear due to a combination of these factors. If Gandalf did pick up a similar blade (might have been a different kind altogether), he certainly knows how to prevent it from disappearing and can maintain its state to study it later, as he is a powerful Maiar.