sar (pl. *sarni) is a noun meaning "stone" in Quenya.
Sar is unusual in Quenya for being a proper one-syllable noun whose vowel is not compensatorily lengthened in the nominative case. One would expect *sarno > **sár
The Etymologies give the stem of the word as sard- (eg. plural sardi) however in another source, the stem of the name Elessar is given as Elesarno. This, and the Sindarin word sarn, point out that the stem was corrected by Tolkien to sarn-.