The Walls, Foundation and the Precious Stones:

Revelation Chapter 21 lists the precious stones that were included in the foundation of the walls of the great city (the bride of Christ: the church).

12 Foundation Stones

The foundation of the walls included twelve foundation stones, and on each of them the names of the twelve apostles: 

And the wall of the city had twelve foundation stones, and on them were the twelve names of the twelve apostles of the Lamb. Revelation 21:14 (NAS)

12 Types of Precious Stones in Foundation

Intermingled within the foundation stones were other precious stones.  When Peter spoke of the church...with Christ being its cornerstone, Peter also spoke about Christians being built together in this living house:

And coming to Him as to a living stone (Jesus), rejected by men, but choice and precious in the sight of God, ⁵you also, as living stones, are being built up as a spiritual house for a holy priesthood, to offer up spiritual sacrifices acceptable to God through Jesus Christ. ⁶For this is contained in Scripture: "Behold I lay in Zion a choice stone, a precious corner stone and he who believes in Him shall not be disappointed." ⁷This precious value, then, is for you who believe. But for those who disbelieve, "The stone which the builders rejected, this became the very corner stone... 1 Peter 2:4-7 (NAS)

Jesus is the precious corner stone.  In addition, all Christians are as "living stones," being built up as a spiritual house for a holy priesthood..."

Although this may have some relevance to the description in the Revelation, no specific explanation for the precious stones is given.  As the 12 stones represented the apostles, perhaps the 12 types of precious stones represented the prophets?

So then you are no longer strangers and aliens, but you are fellow citizens with the saints, and are of God's household, ²⁰having been built upon the foundation of the apostles and prophets, Christ Jesus Himself being the corner stone... Ephesians 2:19-20 (NAS)

12 Precious Stones of the Foundation Revelation 21:19-20
Stone	Example	Stone also mentioned elsewhere in the Bible:
Jasper		Included in the fourth of four rows in the breastplate of judgment (see Exodus 20:20).  The jasper stone was also one of 10 stones clothing the king of Tyre in Ezekiel 28:13. In Revelation 4:3, the One sitting on the throne was like a jasper stone and sardius in appearance.
Sapphire		In Exodus 24:10, Moses and Aaron and 72 men saw a pavement as of sapphire before the Lord, as clear as the sky itself. It was included in the second of four rows in the breastplate of judgment (see Exodus 20:18).  In Ezekiel 10:1, the prophet saw in his vision an expanse over the heads of angels like a sapphire stone.
Chalcedony		This stone does not appear elsewhere in the Bible.
Emerald		Included in the first of four rows in the breastplate of judgment (see Exodus 20:17). The emerald stone was also one of 10 stones clothing the king of Tyre in Ezekiel 28:13.  The rainbow around the throne in Revelation 4:3 was like an emerald in appearance.
Sardonyx		This stone does not appear elsewhere in the Bible.
Sardius		In Revelation 4:3, the One sitting on the throne was like a jasper stone and sardius in appearance.
Chrysolite		This stone does not appear elsewhere in the Bible.
Beryl		Included in the fourth of four rows in the breastplate of judgment (see Exodus 20:20). The husband's hands in Song of Solomon 5:14 are described as like beryl.  In Ezekiel 1:16, the four wheels in his vision were like sparkling beryl. The beryl stone was also one of 10 stones clothing the king of Tyre in Ezekiel 28:13.  Beryl was also the color of the man's body in Daniel's vision recorded at Daniel 10:6.
Topaz		Included in the first of four rows in the breastplate of judgment (see Exodus 20:17). The topaz stone was also one of 10 stones clothing the king of Tyre in Ezekiel 28:13.
Chrysoprase		This stone does not appear elsewhere in the Bible.
Jacinth		Included in the third of four rows in the breastplate of judgment (see Exodus 20:19).
Amethyst		Included in the third of four rows in the breastplate of judgment (see Exodus 20:19).

Note: As noted above, seven of the twelve stones were also used in the breastplate of judgment that Aaron and his sons wore as priests in the Old Testament.  The breastplate had 12 stones; four rows of stones, three stones in each row (see Exodus 20:17-20).

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