The Gates of Pearl

Revelation 21:12-13 & 21 
(12) And had a wall great and high, and had twelve gates, and at the gates twelve angels, and names written thereon, which are the names of the twelve tribes of the children of Israel:  (13)  On the east three gates; on the north three gates; on the south three gates; and on the west three gates. (21) And the twelve gates were twelve pearls; every several gate was of one pearl: and the street of the city was pure gold, as it were transparent glass.