"they grew broader and clearer, until their design could be guessed."
Gate of Moria
Public DomainGate of Moria

The western door of Moria is made from ithildin (see glossary).  Gandalf knew the necessary invocation by which the ithildin would be revealed.  The gates were made by the great Dwarf craftsman, Narvi, with the help of his friend the Elven artificer, Celebrimbor.