Reminder: this string began with an explanation, here.
Rabbi Yochanan was expounding on the future glory of Jerusalem: the Lord will bring jewels of 30X30 Amah (roughly a foot), and will cut and polish them to 10X20 gems, and place them in the gates of Jerusalem. One of the students scoffed: who has ever heard of jewels of such size? Later, the student was sailing on the sea and saw angels in the heavens polishing 30X30 jewels into 10X20 gems. What are the gems for, he asked, and was told that in the future the Lord would have them set in the gates of Jerusalem. He returned to the Yeshiva he told Rabbi Yochanan what he had seen, and that Rabbi Yochanan had been right. "You empty (useless?) fool [the original uses one devastating word: reika], if you hadn't seen for yourself you wouldn't have believed? You allow yourself to mock the words of the scholars?" Immediately he penetrated him with his eyes and turned him into a pile of bones.
Baba Batra, 75a