A word for “necklace” in the name Sigil Elu-naeth “Necklace of the Woe of Thingol” in Silmarillion notes from the late 1950s (WJ/258).
Conceptual Development: The Gnomish Lexicon of the 1910s had G. fring “carcanet, necklace” (GL/59), an element in the early name G. Nauglafring “Necklace of the Dwarves” (LT2/221). ᴱN. fring “necklace” reappeared in Early Noldorin Word-lists of the 1920s (PE13/143), but in Silmarillion drafts of the 1930s the “Necklace of the Dwarves” was renamed to Nauglamír (SM/135), a name it retained thereafter (S/114). The element mîr in the later name means “jewel” (Ety/MIR; LotR/1115), and Tolkien coined a new word sigil for “necklace” in the 1950s, as noted above.
A noun in The Etymologies of the 1930s, the cognate of ᴹQ. sikil “dagger, knife”, both derived from the root ᴹ√SIK (Ety/SIK).
Conceptual Development: The Gnomish Lexicon of the 1910s had G. crî “a knife” (GL/27), perhaps based on the early root ᴱ√KIRISI (QL/47). In Tolkien’s later writings, S. sigil was glossed “necklace” (WJ/258), but there were no later “knife” words. If this conflict bothers you, you might use the neologism ᴺS. cerf for “knife” instead, a cognate of Q. cirma “knife” from 1969.