And you, son of man, take yourself a brick, place it before you, and engrave upon it a city – Jerusalem – …and lay siege against it. This shall be a sign to the house of Israel. (Ezekiel 3:1,3)
A sign made by a prophet indicates that the prophecy will come true regardless of a change of behavior on the part of the people involved (Derashot HaRan). Thus in prophesying that Israel would vanquish the king of Aram, Elisha instructed the king of Israel to shoot arrows from his house in the direction of Damascus (II Kings 13:15-17). The sign ensured that Israel would defeat Aram even if the Arameans were to repent and the Israelites to sin.
Why did G-d ask Ezekiel to represent Jerusalem with a brick when the city was built of stone?
A stone that is broken remains broken. But a brick, before it is fired in the kiln, can be restored. G-d asked Ezekiel to represent Jerusalem by a brick to show that Jerusalem will eventually be restored.