I saw the Quenya word for ruby, serciril, and thought, hey, this could be my username on this site, so I was wondering what the Sindarin equivalent would be—matching the literal name meaning ("blood-glitter") or having a simpler meaning, like red gem or something.
Seregril would be Blood Glitter using sereg + ᴹ√RIL. The word agar “blood” is more obscure than sereg “blood”.
Seregril is what I would suggest too, as a direct cognate of Serciril. Though if this is to be taken as a lexical item of either language (i.e. a common word), then it cannot be used as a name on its own, because Elves do not use simple words as names. You can add a suffix, e.g. (masc.) Seregrillon or (fem.) Seregrilleth.
As for caran+mîr, I would write the result as Carammir to clarify the location of the stress, though of course the 'm' is not actually geminated; it's pronounced [ka.ˈra.mir]. Alternatively it could be Ceremmir with i-affection.
(Also note that caran must have had a final vowel in its primitive form, which prevented the 'n' from dropping out; thus the development Ufgor showed works. If, however, the primitive form really was ✶karan - which it is not - then this would develop into caranwir/cerenwir instead.)
Yes, I should have shown the whole process karanimīri > karanemīre > karanemīr > karanmīr > karanmir > karammir. Ellanto when do you know when it is geminated vs not? Also why it would not shorten like damen? Cause that would be 'ka.ra.mir or 'kɛ.rɛ.mir correct. Which would move the stress to the the first syllable correct? Of course 'da.mɛn is only a two syllable so this might make the difference?
[m] is always short (ungeminated) in Sindarin, no exceptions. However, an intervocalic [m] still counts as a long consonant for stress purposes. For this reason it can be spelled as 'mm' in cases where the stress might be ambiguous. As Tolkien put it in Appendix E:
Tolkien himself didn't follow this very well though... But I prefer to follow the logic he stated. I would also retain 'mm' that arises in conjugations, for clarity.
So both Caramir and Carammir are pronounced exactly the same, both with a short [m], and both with stress on the second syllable; it's just that the latter is slightly more transparent about the position of the stress.