√MBELEK “mighty, powerful, strong; power as force or strength; great, large”
The most notable uses of this root were as the basis for the name Q. Melkor and (sometimes) the adjective S. beleg “great”. This root first appeared as ᴱ√mbelek, belek or melek in the Gnomish Lexicon of the 1910s, with Gnomish name G. Belcha vs. Qenya name ᴱQ. Melko; its other derivatives indicate the meaning of the root was “*flame” (GL/20). The Qenya noun ᴱQ. velka “flame” indicates that unstrengthened ᴱ√belek was used along with strengthened ᴱ√mbelek, but there are no obvious derivatives of ᴱ√melek in this period.
There is no evidence of this root in The Etymologies of the 1930s; in this period ᴹQ. Melko was derived from ᴹ√MIL(IK) “*greed, lust” (Ety/MIL-IK). The root appears a number of times in Tolkien’s later writings, always as the basis for Q. Melkor. The root melk- was mention in notes associated with the essay Athrabeth Finrod ah Andreth from around 1959, where Tolkien said it “means ‘power’ as force and strength” (MR/350). The root appears as √MELEK “great, mighty, powerful, strong” (rejected) or mbelek “large, great” in notes from the late 1950s or early 1960s, connected to both S. beleg and Q. Melkor (PE17/115). The root √MBELEK is implied by Tolkien’s etymology of Q. Melkor in the Quendi and Eldar essay from 1959-60, being derived from ✶Mbelekōre “He who arises in Might” (WJ/402).
It seems Tolkien vacillated on whether Q. Melkor and S. beleg were related. The root may have been √MELEK, unrelated to √BEL. Alternately, it may have been √MBELEK, but the various mutations of S. beleg “great” show no signs of primitive initial mb-. Thus, it seems the strengthening to mb- either occurred only in Quenya, or it enhanced the meaning of the root from “large, great” to “powerful, mighty”. For purposes of Neo-Eldarin, I prefer the last of these theories.
ᴹ√MBIRIL “*crystal”
An unglossed root in The Etymologies of the 1930s as a combination of ᴹ√MIR and ᴹ√RIL, with derivatives like ᴹQ. miril “shining jewel” and Ilk. bril “glass, crystal” (Ety/MBIRIL). The latter was an element in the name Ilk. Brilthor “Glittering Torrent” (LR/263). Tolkien’s continued use of the name S. Brilthor in later versions of The Silmarillion hint at its continuing validity (S/123; WJ/13), though it could be a remnant of earlier ideas that Tolkien never re-examined.
√MBON “*lump, mass”
In a discussion of the comparative written around 1965, Tolkien proposed two new roots to serve as the basis for Q. ambo and S. amon “hill”, first √MAB “lump, mass” > Q. ambo (PE17/90), then √MBŎNO [unglossed] > S. amon, with the Q. form being umbo(n) “hill, lump, clump, mass”, the latter attested nowhere else (PE17/93). Elsewhere “hill” words were typically derived from √AM “up” (PE17/92; Ety/AM²), and Tolkien’s decision to introduce new roots for hill words was probably motivatived by his decision in these 1965 comparative notes to make √AMA = “addition, increase, plus” be the basis for the comparative (PE17/91).
For purposes of Neo-Eldarin, I prefer to assume the comparative is based instead on an-, but √MBON “*lump, mass” may be worth retaining as an additional influence for hill words.
ᴹ√(M)BUD “jut out, project”
This root appeared in The Etymologies of the 1930s as both unstrengthened ᴹ√BUD “jut out” and strengthened ᴹ√MBUD “project”, the latter with derivatives like ᴹQ. mundo and N. bunn “snout, nose; cape [of land]”, as well as ᴹQ. andamunda/N. annabon “elephant, (lit.) long-snouted” (Ety/BUD, MBUD).