The city and its street are pure gold like transparent glass (21:18b, 21b). Transparent gold signifies the nature of God. Bible teachers generally agree that in typology gold signifies the divine nature, the divine essence.
The twelve foundations of twelve different precious stones, bearing the names of the twelve apostles, signify all the New Testament saints, represented by these twelve apostles (21:14, 19-20). The New Jerusalem is a composition of all the redeemed saints, both Old and New Testament. The twelve apostles represent the New Testament saints, while the names of the twelve tribes on the twelve gates represent the Old Testament saints (21:12-13, 21a).
Pearls signify the saints produced through the incarnated, crucified, and resurrected Christ. In His incarnation He was like a living oyster in the death waters. Then He was wounded by us, the little particles of sand. These wounds caused Him to secrete His life juice around us, thus making us into pearls. Here is incarnation, crucifixion, and resurrection. Through this process we, the grains of sand, have been made into precious pearls.
The foundation and the wall built with precious stones signify the saints transformed by the sanctifying Spirit (21:19-20, 18a, 11b). We were made of dust, but we have been regenerated into stone and transformed into precious stones. Dust, regenerated into stone and transformed into precious stone, is qualified to be used as God's building material.
The length, the breadth, and the height of the city are equal, just as the Holy of Holies in the Old Testament had three equal dimensions (1 Kings 6:20). The measurement there is twenty cubits long, twenty cubits wide, and twenty cubits high. This is the Holy of Holies in the type of the temple. The Holy of Holies in the tabernacle is ten cubits by ten cubits by ten cubits (Exo. 26:2-8). In both cases the three dimensions are equal. The principle is that such a building of three equal dimensions signifies the Holy of Holies, which is the very place where God dwells (Rev. 21:16). The entire city, then, is the very place of God's dwelling.