"And the twelve gates were twelve pearls, each of the gates made of a single pearl, and the street of the city was pure gold, like transparent glass." (ESV Rendering)
Compare Rev. 21:18.
John uses ὅμοιον ὑάλῳ καθαρῷ in Rev. 21:18; ὡς ὕαλος διαυγής occurs in Rev. 21:21.
E.W. Bullinger on Rev. 21:21: "as it were. Not that it is glass, but gold of a kind unknown to us."
Robertson's WP: "Transparent (διαυγής — diaugēs). Old word (from δια — dia through, αυγη — augē ray, shining through), here alone in N.T."
Peter Pett's Commentary:
"'And the twelve gates were twelve pearls, each one of the several gates was one pearl, and the street of the city was pure gold, as it were transparent glass.'
In Matthew 7:6 pearls represented what was holy and precious, compare the pearl of great price (Matthew 13:46). Gold again is symbolic of the holy sanctuary, where all is made of gold. The transparency may well denote total openness and honesty. The city contains all that is most splendid. We can compare many of these splendours with those which poured into Babylon at its finest (Revelation 17:4; Revelation 18:12), but here it is heavenly gold, heavenly jewels and heavenly pearls of a size unknown to earth.
But in the new creation such things as gold and precious stones in their literal senses will be meaningless. They are used as descriptions here only because of fallen man’s peculiar propensities. They denote what is better far than gold. Note that only one street is mentioned and yet there are twelve entrances. As there are no buildings the whole inside may be intended to be seen as the street. The point is that all is of gold. (Not liveable in but splendid in conception)."
When explaining Rev. 21:18-21, Dr. Thomas Constable invokes 1 Kings 6:30.
Cf. 1 Cor. 3:12.
Contrast John's description of the New Jerusalem with how he saw Babylon the Great in Rev. 17:4.
Rev. 18:12 mentions gold, precious stone, and pearl, among other things.
Rev. 18:16 likewise refers to gold, precious stone, and pearls.