This root and ones like it were used for Elvish words for “silver” throughout Tolkien’s life. The earliest iteration of the root began with T-, however, appearing as unglossed ᴱ√TELEPE in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. telpe “silver” (QL/91). Even at this early stage, however, the Gnomish equivalent was G. celeb (GL/25), but the reason for the t/c variation isn’t clear. The closest explanation is that palatal consonants like [c] became [tʲ] in Qenya vs. [k] in Gnomish (compare ᴱQ. tyava- vs. caf- “taste” from ᴱ√TYAVA) but this doesn’t explain why the Qenya form has initial t- rather than ty-.
Elsewhere in the Elvish languages of the 1910s there seem to be etymological variations of [k] vs. [t], such as ᴱQ. kitya- vs. G. tisca- “tickle” (QL/47; GL/70) and ᴱQ. talqe vs. G. celc “glass” (QL/88; GL/25), so perhaps ᴱQ. telpe vs. G. celeb “silver” is another example of this. Another explanation appeared in Early Noldorin word lists from the 1920s, where the primitive form was ᴱ✶kelekwé which produced ᴱN. celeb as usual but the Qenya form was ᴱQ. telqe with “k = t by dissimilation” (PE13/140), presumably away from q.
In The Etymologies of the 1930s Tolkien had the root ᴹ√KYELEP with variant ᴹ√TELEP, producing N. celeb but ᴹQ. tyelpe or ᴹQ. telpe (Ety/KYELEP). But Tolkien revised this entry, marking ᴹ√TELEP as questionable and introducing the Telerin form ᴹT. telpe < ᴹ√KYELEP, concluding that ᴹQ. telpe must be a loan from Telerin. This finally put N. celeb vs. ᴹQ. telpe (borrowed from Telerin) on a solid phonological foundation. Tolkien seems to have stuck with this explanation, mentioning this borrowing from Telerin to Quenya several times in his later writings, with the proper but now archaic Quenya form being Q. †tyelpë (Let/426; PM/356; UT/266).
Tolkien frequently used the root √THIL as the basis for “moon” words in Elvish. The first appearance of this root was ᴱ√ÞILI a variant of ᴱ√SILI “✱gleam” from the Qenya Lexicon of the 1910s (QL/83). The normal “moon” words in this period were ᴱQ. Sil and G. Sil (QL83; GL/67). A rejected Gnomish variant Thil “the moon” indicates Tolkien considered ᴱ√ÞILI, but did not commit to it (GL/72). However, in The Etymologies of the 1930s Tolkien gave unglossed ᴹ√THIL with derivatives like ᴹQ. Isil and N. Ithil “Moon, (lit.) Sheen” (Ety/I²; THIL). Tolkien stuck with these “Moon” words thereafter, and the root √THIL was mentioned regularly in Tolkien’s later writings (Let/425; PE17/66; PE22/136).