🧙Gandalf's Thoughts....

[Maeglin]

Gandalf moved his chair to the bedside, and took a good look at Frodo. The colour had come back to his face, and his eyes were clear, and fully awake and aware. He was smiling, and there seemed to be little wrong with him. But to the Wizard's eye there was a faint change, just a hint as it were of transparency, about him, and especially about the left hand that lay outside upon the coverlet. "Still that must be expected," said Gandalf to himself. "He is not half through yet, and to what he will come in the end not even Elrond can foretell. Not to evil, I think. He may become like a glass filled with a clear light for eyes to see that can."

This is very soon after Frodo awakes after his incident at the Ford with the Nazgul, but what in the world is Gandalf thinking about, or what are his thoughts referring to?

 

[Confusticated]

hmm... well I take the transparency just to mean that part of Frodo's spirit was damaged or lost to the wraith world, and that Gandalf was thinking that if this were not healed (or even if he were) that when he eventually died he would be a spirit of light in the world of the dead, sort of the way Glorfindel (and all calaquendi) showed up as white figures in the world of shadows. If Frodo fell to evil his spirit wouldn't be a white light as Glorfindel's was back dark like the wraiths, and Gandalf was thinking that this would not happen.

But this is just my take on it, and it might be wrong.

 

[Lantarion]

I see it as that the wound given to Frodo by the Nazgûl blade could not be completely healed, but that it was patched up so well that despite his fading it would take little or no toll on him because his spirit and his soul would 'fill' the emptiness.

 

[Arvedui]

Come to think of it:

Gandalf may have been talking in methaphors, comparing Frodo with a Silmaril.
Considering what Frodo accomplished, this wouldn't be too stupid a comparison, after all.

 

[Ithrynluin]

This has also been discussed here.