"Having the glory of God: and her light was like unto a stone most precious, even like a jasper stone, clear as crystal; 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: On the east three gates; on the north three gates; on the south three gates; and on the west three gates. And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb." (Revelation 21:11-14)
The New Jerusalem is the heavenly city for which every true believer waits. She is, as Paul put it, "the mother of us all;" and the principles of redemption for which she stands are in direct opposition to the flesh-based religions of mankind (Galatians 4:25-26).
John now begins to describe with some detail the physical beauty of the city. Every aspect of her grandeur reflects the purity and splendor of Israel's Holy One. A jasper stone can be red, yellow, brown or green and, in some instances, blue. The city emitted beams of light which, to John, resembled the colors of a transparent jasper. Such an image of God's glory would be dazzling. Quite likely, John's description falls far short of the actual reality. Comparisons help, but they are not complete substitutes.
Like the millennial Jerusalem (Ezekiel 48:31-34), the city's gates stand in memorial of Israel's twelve tribes. Dan was excluded from having a place among the 144,000 witnesses; but here he is likely reinstated since his name is definitely found on the gates of the millennial Jerusalem. God's grace is that which reinstates a man when he deserves to be excluded.
The gates bring to remembrance the old covenant while the foundations of the city, being laid in memorial of the Lord's twelve apostles, call to remembrance the new. In this construction, the inseparability of the Old and New Testaments is seen. God's dealings with Israel and His workings through the Church are not mutually exclusive. The entire Bible is cohesive. Israel is not the Church, and the Church should never replace Israel; but God's holiness, love and redemption have not changed between Covenants.
For I am the LORD, I change not...(Malachi 3:6).