The Elvish words for “stone” were established very early as Q. ondo and S. gond. In the Qenya Lexicon of the 1910s Tolkien gave the root of these words as ᴱ√ONO “hard” with derivatives like ᴱQ. ondo “stone, rock” and ᴱQ. onin “anvil” (QL/70). But its Gnomish derivatives like G. gonn “stone” and G. gontha “pillar” (GL/41) indicate the actual root was ✱ᴱ√ƷONO, since initial ʒ > g in Gnomish.
In The Etymologies of the 1930s Tolkien gave the root as ᴹ√GONOD or √GONDO “stone” with essentially the same Elvish forms: ᴹQ. ondo and N. gonn (Ety/GOND). The root itself did not appear in later writings, but Tolkien continued to state, with great frequency, that the primitive form of the word was ✶gondō (Let/410; PE17/28; PE18/106; PE21/81; PM/374; RC/347).
A root in The Etymologies of the 1930s serving principally as the basis for N. brethil “beech” (Ety/BERÉTH). In the notes from the late 1950s or early 1960s Tolkien instead connected brethil to the root √BARATH and S. bereth “queen”, where it had the true meaning of “princess” (PE17/23). But that could be a later association rather that a genuine etymology, and doesn’t necessarily invalidate ᴹ√BERETH (especially given the other phonological issues with S. bereth < √BARATH).