The full and proper title for The Silmarillion (S/33). It is a combination of quenta “tale” and the genitive plural of Silmaril.
Conceptual Development: In the early 1930s, the original Quenya title of the cycle of tales that eventually became The Silmarillion was ᴹQ. Qenta Noldorinwa (SM/77). In the mid-30s, Tolkien introduced the name ᴹQ. Qenta Silmarillion which became the full title of the tales, while Quenta Noldorinwa became an alternate title (LR/202).
The name of the great jewels crafted by Fëanor from which The Silmarillion legends gets their name (S/33). It sometimes appeared in the longer form Silmarillë (PE17/47, PM/363). Tolkien translated this name as “radiance of pure light” (Let/148) and said that it was a combination of silima, the crystaline substance from which the jewels were made (RGEO/65), and a derivative of the root √RIL “brilliant, brilliance” (PM/363), perhaps in the form [ᴹQ.] rille “brilliance”.
Conceptual Development: The name ᴱQ. Silmaril dates back to the earliest Lost Tales (LT1/128). Christopher Tolkien suggested that at this early stage it was a compound of ᴱQ. Sil “Moon” and ᴱQ. marilla “pearl” (LT1A/Silmaril), which is consistent with its Gnomish cognate G. Silubrilt. In The Etymologies, the name ᴹQ. Silmaril appears as a combination of silma “silver, shining white” and the root ᴹ√RIL (Ety/RIL, SIL), similar to its later etymology.