R. Johanan said that the future Eastern portal and posts of Jerusalem would be made of one pearl. A Min scoffed at it. After a time he travelled in a ship and saw in the sea the angels sawing and preparing the pearl for the gate. On his return he confessed it to R. Johanan, who looked at him and turned him into a heap of bones.
The Gate of Jerusalem Made of a Single Pearl
Gaster, Exempla of the Rabbis No. 203 (1924); Bava Batra 75a