§ Elements in Quenya and Sindarin Names The Silmarillion

sil- (and variant thil-) 'shine (with white or silver light)' in Belthil, Galathilion, Silpion, and in Quenya Isil, Sindarin Ithil, the Moon (whence Isildur, Narsil; Minas Ithil, Ithilien). The Quenya word Silmarilli is said to derive from the name silima that Fëanor gave to the substance from which they were made.