on, ondo noun "stone" (LT2:342, LT1:254 probably only ondo in LotR-style Quenya, see below). Various "Qenya" forms: ondoli "rocks" (MC:213; this would be a partitive plural in LotR-style Quenya), ondolin "rocks" (MC:220), ondoisen "upon rocks" (MC:221), ondolissen "rocks-on" (MC:214; the latter form, partitive plural locative, is still valid in LotR-style Quenya).
Quenya
ondo
noun. stone (as a material), (large mass of) rock
on
stone
ondo
stone
ondo noun "stone" as a material, also "rock" (UT:459, GOND). Pl. ondor in an earlier variant of Markirya; partitive pl. locative ondolissë "on rocks" in the final version. Compounded in ondomaitar "sculptor in stone" (PE17:163), Ondoher masc.name, *"Stone-lord" (ondo alluding to Ondonórë = Sindarin Gondor, "stone-land") (Appendix A), #ondolunca ("k") "stonewain", possessive form in the place-name Nand Ondoluncava "Stonewain Valley" (PE17:28, also Ondoluncanan(do) as a compound). Ondolindë place-name "Gondolin" (SA:gond, J.R.R. Tolkien: Artist & Illustrator p. 193); see Ondo. Earlier "Qenya" has Ondolinda _(changed from Ondolin) "singing stone, Gondolin" (LT1:254)_
ʼondō
noun. stone
PQ. stone
The basic word for “stone” in Quenya. More specifically, it was “stone as a material, a large mass of stone or great rock” (RC/347; PE17/28; Ety/GOND) as opposed to a small individual stone, which was Q. sar (Ety/SAR). Ondo was derived from the root ᴹ√GONOD of essentially the same meaning, as was its Sindarin cognate S. gond (Ety/GOND).
Conceptual Development: The earliest iteration of this word was ᴱQ. on(d) “a stone” in the Qenya Lexicon of the 1910s under the root ᴱ√ONO “hard” (QL/70); it was ond(o) in the contemporaneous Poetic and Mythological Words of Eldarissa (PME/50). Its Gnomish cognate was G. gonn “great stone, rock” (GL/41) meaning the early root form was probably ✱ᴱ√ƷONO, with the initial ʒ vanishing in Qenya but becoming g in Gnomish. Later on, this derivation no longer worked, since Tolkien decided that initial ʒ became h in Qenya, as reflected in the form ᴱQ. {ŋonda >>} hond- “stone, rock” in the Early Noldorin Dictionary of the 1920s (PE13/162).
However, the usual form became ondo as of the Early Qenya Grammar and various word lists in the 1920s (PE14/43-44; PE15/77; PE16/138), and ᴹQ. ondo “stone (as a material)” appeared in The Etymologies of the 1930s, already with the derivation given above (Ety/GOND). Tolkien stuck with this form thereafter.