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-.
Elessar masc. name "Elf-stone" (Elen + sar, actually "Star-stone", cf. Elendil concerning elen "star" being used to mean "Elf") (LotR3:V ch. 8). Genitive Elesarno _(VT49:28, read _Elessarno?) indicates that the stem is -sarn-. As a common noun, elessar or "elf-stone" may signify "beryl" (in the chapter Flight to the Ford in the LotR, Aragorn finds "a single pale-green jewel" and declares: "It is a beryl, an elf-stone"). Elessar as a name may also be seen as a pun or variant of Elesser "Elf-friend".