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 used the root √SIL and its variant √THIL for Elvish words for shining things for most of his life. This root first appeared in the Qenya Lexicon as unglossed ᴱ√SILI with variant ᴱ√ÞILI and derivatives like ᴱQ. Sil “Moon”, ᴱQ. sili- “gleam, glint”, ᴱQ. silik “flint”, and ᴱQ. silqe “glossy hair” (QL/83), though last of these was the result of blending with ᴱ√SḶKḶ “rich, lush”, variant of ᴱ√SṚKṚ “fat” (QL/86). The contemporaneous Gnomish Lexicon had derivatives like G. Sil “Moon” and G. silc “flint” pointing only to ᴱ√SILI (GL/67); the only indication of variant ᴱ√ÞILI was in unglossed G. thil followed by deleted forms like G. Thil “Moon” (GL/72).
By The Etymologies of the 1930s, the variants ᴹ√SIL “shine silver” and ᴹ√THIL were better established (Ety/SIL, THIL). The variant ᴹ√SIL had derivatives like ᴹQ. silme/N. silif “moonlight, light of Silpion” and the extended form ᴹ√SÍLIP from which ᴹQ. Silpion was derived (Ety/SIL), and its most notable use from the 1930s forward was as the basis for the initial element of Q. Silmaril vs. S./N. silef as in N. Silevril (Ety/SIL; PE17/23; Let/425). The variant ᴹ√THIL had derivatives like ᴹQ. Isil/N. Ithil “Moon, (lit.) the Sheen” (Ety/THIL), forms Tolkien also retained from the 1930s forward. Tolkien mentioned both root variants √SIL and √THIL with the glosses like “shine (white or silver)” regularly in his later writings (Let/425; PE17/66; PE22/136; SA/sil).