A word for “stone”, most notably in the name Q. Elessar “Elfstone” (LotR/375). The genitive for that name was given as Elesarno (PE22/158; VT49/28), so the stem for this word is probably sarn-, especially given its better-attested Sindarin cognate S. sarn. It is a derivative of the root ᴹ√SAR, and referred to a small (individual) stone (Ety/SAR), as opposed to Q. ondo which is stone as a material or large rocks or bodies of stone.
Conceptual Development: The earliest hint of this word was ᴱQ. sar “earth, soil” in Early Qenya Word-lists of the 1920s, but this was marked with an “X” and ᴱQ. kemen (of similar meaning) was written next to it as an alternative (PE16/139). ᴹQ. sar “(small) stone” appeared in The Etymologies of the 1930s derived from the root ᴹ√SAR, but there it had a plural sardi and hence a stem form of sard- (Ety/KEM). In that document, N. sarn was “stone as material” and hence not directly related. But later S. sarn was used for individual stones, and thus was probably a direct cognate of Q. sar (sarn-), as discussed above.
sar (sard-, as in pl. sardi) noun "(small) stone" (SAR). In Elessar, q.v. Since Tolkien let this name have a stem in -sarn- (genitive Ele[s]sarno, VT49:28), he may seem to have changed the stem-form of sar from sard- to sarn-.