The future Jerusalem will be built from precious stones so massive they defy imagination. According to Bava Batra 75a, God will take gems thirty cubits by thirty cubits, carve gates from them ten cubits by twenty cubits, and set them in the walls of the city.
A student once mocked this teaching. "Gems that size do not exist," he said. "We cannot even find jewels the size of a dove's egg." Some time later, the student was traveling at sea and saw ministering angels sitting and sawing precious stones of exactly those dimensions—thirty by thirty cubits—with openings of ten by twenty. He asked: "For whom are these?" The angels answered: "The Holy One, Blessed be He, will set them in the gates of Jerusalem." The student returned and told his teacher: "Teach on, my master. It is fitting for you to teach, for I have seen exactly what you described." The teacher replied: "Empty one—if you had not seen it, you would not have believed it? You mock the words of the Sages!" He fixed his gaze on the student, and the student became a pile of bones.
The same passage teaches that Jerusalem of the World to Come will not be like Jerusalem of this world. In this world, anyone who wishes can ascend to Jerusalem. In the World to Come, only those who are invited.
Rabbi Yohanan taught that God will raise Jerusalem three parasangs into the sky (Zechariah 14:10). And to prevent the climb from being difficult, the righteous will fly there—"Who are these that fly as a cloud, and as the doves to their cotes?" (Isaiah 60:8).
Rabbi Shmuel bar Nahmani added that three things will be called by the name of God: the righteous, the Messiah, and Jerusalem itself. The city is not merely a location. In the age to come, it will bear the divine name.