A root appearing in an undated note from around 1968 as part of an explanation for the tree-name S. lebethron, given as {√RON >>} √RUN “rub, grind, smooth, polish”, with a derivatives Q. runda “smooth, polished” and [deleted] ron “smoothed by polishing”, probably Sindarin (PE17/89).
The Etymologies of the 1930s instead had the root ᴹ√RUN “flat of hand or sole of foot” with derivatives like ᴹQ. runya/N. rhein “footprint”, and also as the basis for the final element of ᴹQ. tallune/N. tellen “sole of foot” (Ety/RUN). It’s conceivable the two iterations of the root are related based on the “smoothness” of the hand or foot.
A root in Quenya Notes (QN) from 1957 glossed “solid, tangible, firm” with derivative Q. ronda “solid, firm” (PE17/183). It had a couple variants: √GRON > S. grond “solid, tangible, firm”, also the name of Melkor’s hammer, as well as √SRON > Q. hrón/S. rhû “matter” and Q. hrondo/S. rhond “body”. The root √SRON appeared again with the gloss “flesh, substance, matter” in etymological notes associated with the essay Of Death and the Severance of Fëa and Hrondo from the late 1950s, along with both Q. hrón “matter” and Q. hrondo “physical body” (MR/231 note #26). However, in the title of this essay hrondo was revised to Q. hröa (MR/217), elsewhere derived from primitive ✶srawā (MR/350); see the entry on √SRAW for further discussion (MR/330).
Regarding S. grond: in The Etymologies of the 1930s this word was derived instead from the unglossed root ᴹ√RUD, which had derivatives ᴹQ. runda “rough piece of wood” and ON. runda > N. grond “club” (Ety/RUD); it also had strengthened variant ᴹ√G-RUD (EtyAC/G-RUD).
Neo-Eldarin: Despite the change of Q. hrondo >> Q. hröa, I think √SRON might be retained in the more limited sense “matter” rather than “flesh”, along with related root √(G)RON “solid” from 1957. As for ᴹ√RUD, it might also be retained, with S. grond [N. grond] “weighty; club” being a blending of ᴹ√RUD and √(G)RON.