After doing much research I came across
The Quettaparma Quenyanna Wordlist. According to to this site
Precious Thing is mírë... but that isn't plain precious. I do realize this is probably not of much help, but I felt I should inform you of what I found anyway. It might help?
I looked through numerous sites but for some odd reason none of them had precious. It is funny seeing precious is one of the most commonly used words used in Lord of the Rings.
God Bless,
Niluial