This word is indeclinable, according to the Etymologies
Noldorin
coe
noun. earth
amar
noun. Earth
amar
noun. earth
ambar
noun. earth
coe
noun. earth
emerin
place name. Middle-earth
A Noldorin name for “Middle-earth” appearing in The Etymologies as a cognate of ᴹQ. Ambarenya (Ety/MBAR), apparently derived from the same primitive form ✱✶ambarenyā.
cevn
adjective. of earth, earthen
A Noldorin adjective in The Etymologies of the 1930s glossed “of earth, earthen” and given as a derivative of the root ᴹ√KEM (Ety/KEM). Hiswelókë’s Sindarin Dictionary suggested an alternate (Neo) Sindarin spelling cefn (HSD/cefn) which is more than in keeping with Tolkien’s usual spelling conventions from the 1930s, but I prefer cevn as it makes the pronunciation clearer; see the discussion of the Sindarin phoneme [v] for more details.
barthan
masculine name. Earth-smith, Earthbuilder
bár
noun. home; earth
cef
noun. soil
A noun in The Etymologies of the 1930s glossed “soil” derived from the root ᴹ√KEM “soil, earth” (Ety/KEM). It had a rather unusual plural form ceif which shows *i*-intrusion but not the usual *i*-raising of *e* to *i*; under ordinary phonetic developments the expected plural would be ✱cif. I can think of no good reason for this abnormal plural, but it would be ^caif if adapted to Sindarin. It is mostly moot since this word is very unlikely to be used in the plural.
cef
noun. soil
gwatha-
verb. to soil, stain
hmaw
noun. soil, stain
hmaw
noun. soil, stain
lith
noun. sand
maw
noun. soil, stain
An indeclinable word given as {cíw >>} coe “earth” in The Etymologies of the 1930s as a derivative of the root ᴹ√KEM (Ety/KEM; EtyAC/KEM).
Possible Etymology: The primitive form of rejected cíw is given as ᴹ✶kēm and its derivation is clear: the long *ē* became *ī* and then the final *m* reduced to *w* after *i* as usual. The derivation of coe is more obscure, however. The likeliest explanation is that Tolkien imagined its ancient form with a slightly lowered vowel which he generally represented as ǣ in this period (in later writings as ę̄). According to the first version of the Tengwesta Qenderinwa and Comparative Vowel Tables from the 1930s (PE18/46; PE19/25), *ǣ* > *ei* > *ai* > *ae*, and in The Etymologies itself, it seems *ai* often became *oe* instead of *ae*.
Neo-Sindarin: Updating the derivation of hypothetical ✱kę̄m would produced ᴺS. cae in Sindarin phonology. But given the obscurity of its derivation, I recommend using 1950s S. ceven for “earth” instead.