Ithildin:

A substance made of Mithril. It was only visible under the light of moon and stars after certain words were spoken:
They are wrought of ithildin that mirrors only starlight and moonlight, and sleeps until it is touched by one who speaks words now long forgotten in Middle-Earth. (LOTR.322)
One location ithildin was used was the West Gate of Moria. There are no surviving records of other places using ithildin, though likely the city Ost-in-edhil had used ithildin before it was ruined. This was the city that the Dwarves had created the West Gate to trade with.

The Lord of the Rings
The Silmarillion
The Unfinished Tales
The Hobbit
The Complete Guide to Middle Earth by Robert Foster