And the twelve GATES were twelve PEARLS; each one of the GATES was a single PEARL.
Over the centuries of present earth, the PEARL has been revered as much as precious metals. Many cultures in ancient writings extol the PEARL and its value.
Jesus equated PEARLS to that which is sacred and holy and truthful. On this earth, we are to be careful with whom we share these TRUTHS (Matthew 7:1-6-6). In the new Jerusalem, nothing but truth will reside within the city. One can look at it as though the GATES, although they are never closed, are there to filter out any untruth which may try to enter. The twelve PEARLS visually describe what Jesus said about Himself in John 14:1-6-7-11. Jesus is the WAY (the STREET (singular), here), the TRUTH (the PEARLY GATES, here) and the LIFE (the new Jerusalem is ETERNAL LIFE here). As Jesus said, "...no one comes to the Father but through ME (the GATE). With the names of the tribes of the sons of Israel associated with the GATES (verse 12), every believer should be thankful for our relationship with our Jewish brothers.
And the STREET of the CITY was PURE GOLD, like transparent GLASS.
The Greek word for STREET as used here is Plateia. Some translators indicate rather than being translated as STREET (singular), it actually is more like a wide open space. One can then visualize that this open area can accommodate the River of the water of LIFE (Revelation 22:1), the Tree of LIFE (Revelation 22:2), and the Throne of God (Revelation 22:3). Recall that before the Throne in Revelation 4:1-6 was a sea of GLASS like crystal which fits the description given here. Again the GLORY of GOD gives the STREET (area) a GOLDEN hue of purity and righteousness.
From Physics, we learn that electrons of materials when they change orbital energy levels often emit electromagnetic radiation in the visible range. This is how the Mars rovers, for example, can identify the materials in the Columbia Hills and on the outcroppings around craters. The spectrograph for the STREET in the new Jerusalem emits light (verse 23) that indicate it is like PURE GOLD.