This word is indeclinable, according to the Etymologies
Noldorin
coe
noun. earth
coe
noun. earth
coenel
noun. gore, wedge
coen
?. [unglossed]
-gon
suffix. valour
adwen
noun. [unglossed]
amar
noun. earth
amar
noun. Earth
ambar
noun. earth
borth
?. [unglossed]
caun
noun. valour
caun
noun. valour
caw
noun. top
caw
noun. top
clei
?. [unglossed]
diragas
?. [unglossed]
hûl
noun. cry of encouragement in battle
lhong
adjective. heavy
lhong
adjective. heavy
mauth
?. [unglossed]
menwed
?. [unglossed]
naith
noun. gore
nith
?. [unglossed]
nithrad
?. [unglossed]
níth
?. [unglossed]
porennin
?. [unglossed]
telwen
?. [unglossed]
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.