Ithildin was the special substance made of Mithril that was devised by the Elves. It was magical, and could only be seen by the light of the Moon and stars, but even by their light a password had to be spoken for it's hidden powers to be revealed.

The door of the West-gate of Moria was inlaid with designs made of Ithildin.

Gandalf translated the word Ithildin as 'starmoon' but if interpreted literally, the name would be more akin to 'moonsparkle'.