Tolkien is extending comparative philology to the level of comparative mind reading! Here are two distinct pictures of silent communication: (i) direct looking mind-to-mind (the Lady Galadriel); (ii) reception of the word-thoughts of another (the hobbit, Pippin + 2 Seeing Stones). The second is no less mediated than our spoken words, mixing our spoken and written communication such that the inner thought is communicated through the eye while looking at - not a sign, but - a face. It is surely less 'magical' than the art of Galadriel and is maybe obscurely illuminated by this account of understanding another person through language published in a book of 1938 by R. Collingwood, Tolkien's one time colleague at Pembroke:Two persons, each using a Stone 'in accord' with the other, could converse, but not by sound, which the Stones did not transmit. Looking one at the other they would exchange 'thought' - not their full or true thought, or their intentions, but 'silent speech,' the thoughts they wished to transmit (already formalized in linguistic form in their minds or actually spoken aloud), which would be received by their respondents and of course immediately transformed into 'speech,' and only reportable as such.
When I first read this it also blew my mind - it is nothing like what seems to go on when I listen to someone, and yet it kind of makes sense... Now I'm wondering, if we substitute 'see' for 'hear', does Collingwood spell out an analogy of the communication Tolkien describes with the stones, such that silent thoughts are transferred from one mind to another: in everyday life by spoken sounds, but in the fantastical image of a Seeing Stone, by seeing in the Stone the face of another?The hearer… takes what he hears exactly as if it were speech of his own: he speaks to himself with the words that he hears addressed to him, and thus constructs in himself the idea which those words express. (The Principles of Art (1938) [1958, 250])
(Lockdown thoughts. Edits: tidying up)