Godwin's explorer, a diminutive Spaniard named Gonsales,** finds that the lunar language consists of musical notes, so that they can communicate through singing melodies; he even uses musical notation to depict it [p. 109]. Later, when upon his return to Earth he finds he's landed in China, he suggests a lot of similarities between Lunar and Chinese (e.g., in the latter's emphasis on pitch and tone).
Gonsales also finds a new colour on the moon (shades of Lindsay's jale and ulfire, and of Lovecraft's Colour, and of Bierce):
. . . how to describe the colour of them . . .
It was neither blacke, nor white, yellow, nor redd,
greene nor blew, nor any colour composed of these.
But if you aske me what it was, then I must tell you,
it was a colour never seen in our earthly world,
and therefore neither to be described unto us by any,
nor to be conceived of one that never saw it.
For as it were a hard matter to describe unto
a man born blind*** the difference
betweene blew and Greene, so can I
not bethinke my selfe of any meanes
how to decipher unto you this Lunar colour,
having no affinitie with any other
that ever I beheld with mine eyes.
Onely this I can say of it, that it was
the most glorious and delightfull, that
can possibly be imagined; neither in truth
was there any one thing, that more delighted me,
during my abode in that new world, than
the beholding of that most pleasing and
resplendent colour. [p. 100]
A few hints made me suspect that this may have been one of CSL's source-books for OUT OF THE SILENT PLANET -- Gonsales' discovery that space is filled with light (apparently a common belief during the period), or that he discovers the Lunar inhabitants to be not just good Xians (Godwin was a bishop, after all) but in an apparently unfallen state, with no murders (it's simply too hard to kill each other, given Lunar vitality) and a peaceful acceptance of death when one's time has come. However, it's clear from editor Wm Poole's apparatus to the book (sixty pages of introduction and forty of appendices, bracketing not quite sixty pages of actual text) that Godwin's was part of a tradition, and I don't know enough about that tradition (though I plan to learn more).
One passage of Tolkienian interest was in Gonsales' description of the new types of rocks he found on the moon; here the editor notes that Godwin is drawing on THE BOOK OF SECRETS attributed to Albert Magnus; a reference to the creation of artificial shining stones might be worth following up on. Of particular relevance, though, is the passage about Stones of Invisibility:
I inquired then amongst them, whether they had not
any kind of Iewell or other meanes to make a man
invisible, which mee thought had beene a thing of great
and extraordinary use.
And I could tell of divers of our learned men
had written many things to that purpose.
They answered that if it were a thing faisable,
yet they assured themselves that God
would not suffer it to be revealed to us creatures
subject to so many imperfections, being a thing
so apt to be abused to ill purposes; and that
was all I could get of them. [p. 112]
--all v. Platonic!
current book: THE MAN IN THE MOONE by Francis Godwin 
current audio book: THE DUNWICH HORROR, HPLHS 'radio-play' adaptation (just finished)
*which I suspect I'll start to read my way through, off and on, one by one, prob. stating with Kepler's SOMNIUM 
**referred to on the title page as "Domingo Gonsales, The Speedy Messenger" -- a distant inspiration for Speedy Gonzales, perhaps?
***cf. C. S. Lewis's story of the same name.