The twelve gates were twelve pearls: each individual gate was of one pearl.
Being made of pearl speaks, primarily, of great value (Rev. Rev. 17:4+). Pearls, coming from an unclean animal (an oyster, Lev. Lev. 11:10), may also carry a secondary association of unclean or Gentile. In order to move the gospel out from the Jews to the Gentiles, Peter was given a vision of a sheet containing many unclean animals and told to kill and eat them. When he refused to partake of the unclean animals, God responded, What God has cleansed you must not call common (κοίνου [koinou] ) (Acts Acts 10:13). This was preparatory to Peter entering the household of a Gentile, Cornelius, so that the gospel could go forth to the Gentiles. Here, John is told that nothing that defiles (κοινουν [koinoun] , common ) will be allowed to enter through the gates of the city (Rev. Rev. 21:27+). Jesus paid for both a treasure hidden in a field (Israel) and a pearl of great price (the Gentiles) (Mtt. Mat. 13:45-56). The pearl Jesus bought was considered unclean, but also of great value. The pearls may stand as a testimony that the redeemed from among the Gentiles, who responded to God in faith, are no longer considered unclean and can pass through the gates.
the street of the city was pure gold, like transparent glass
The street of the city is like the sea of glass (Rev. Rev. 4:6+; Rev. 15:2+; Rev. 21:18+). See commentary on Revelation 4:6 and Revelation 15:2.