With what material must every building in Jerusalem be faced?

From ancient times until today, Jerusalem, Israel has been built with what is known as "Jerusalem Stone," a type of limestone found locally. A municipal law requires that all buildings be faced with local Jerusalem stone - this law was put in place in the early 1900s during the British Mandate period. When the sun sets, the limestone buildings of both ancient and modern structures often gives them a golden hue which is the origin of the phrase (and song) "Jerusalem of Gold."