Ithildin
From Tolkien Gateway
A magical substance made by the Elves from Mithril, that could only be seen by the reflected light of the moon and stars, and even then remained hidden until a magical word was said. Gandalf translated its name as 'starmoon', but 'moon-sparkle' would perhaps be a more literal rendering. The designs on the West-gate of Moria were made from this substance.