The interpretation timeline
Revelation 21:18 Douay-Rheims

And the building of the wall thereof was of jasper stone: but the city itself pure gold, like to clear glass.

A.D. 303 – 1871 5 witnesses

Patristic

before A.D. 750
303
A.D.
c. A.D. 250–303
“The city which he says is squared, he says also is resplendent with gold and precious stones, and has a sacred street, and a river through the midst of it, and the tree of life on either side, bearing twelve manner of fruits throughout the twelve months; and that the light of the sun is not there, because the Lamb is the light of it; and that its gates were of single pearls; and that there were three gates on each of the four sides, and that they could not be shut. I say, in respect of the square city, he shows forth the united multitude of the saints, in whom the faith could by no means waver. As Noah is commanded to make the ark of squared beams, that it might resist the force of the deluge, by the precious stones he sets forth the holy men who cannot waver in persecution, who could not be moved either by the tempest of persecutors, or be dissolved from the true faith by the force of the rain, because they are associated of pure gold, of whom the city of the great King is adorned. Moreover, the streets set forth their hearts purified from all uncleanness, transparent with glowing light, that the Lord may justly walk up and down in them.”
Source

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“And the building of the wall of it was of jasper,.... Which is expressive of the impregnableness, duration, brightness, and glory of salvation by Christ, the jasper being a very hard, durable, and bright stone; and salvation can never be made void, and of none effect; it will last for ever, and in this state will come forth as light, and as a lamp that burneth; it is represented by the same precious stone as God and Christ themselves are; see Rev 4:2. And the city was pure gold, like unto clear glass; it was made or built of gold; its parts, buildings, houses, and palaces, were all of gold, and that the best; and it was like to transparent glass; that is, either the city, as most copies read, or the gold; for the Alexandrian copy reads, and so the Vulgate Latin version; the gold of which it was is different from common gold; and as this city, the new Jerusalem, designs the saints, the precious sons of Zion, comparable to fine gold, the inhabitants of the new earth; this denotes the solidity, excellency, and preciousness of them, the rich and happy state they will be in, as well as their purity and freedom from all sin and corruption, and the clear knowledge of things they themselves will have, and others will have of them; their hearts and actions will be open to all; nor will this gold have any rust upon it any more, or ever be changed, and become dim. . And the city was pure gold, like unto clear glass; it was made or built of gold; its parts, buildings, houses, and palaces, were all of gold, and that the best; and it was like to transparent glass; that is, either the city, as most copies read, or the gold; for the Alexandrian copy reads, and so the Vulgate Latin version; the gold of which it was is different from common gold; and as this city, the new Jerusalem, designs the saints, the precious sons of Zion, comparable to fine gold, the inhabitants of the new earth; this denotes the solidity, excellency, and preciousness of them, the rich and happy state they will be in, as well as their purity and freedom from all sin and corruption, and the clear knowledge of things they themselves will have, and others will have of them; their hearts and actions will be open to all; nor will this gold have any rust upon it any more, or ever be changed, and become dim. Revelation 21:19 rev 21:19 rev 21:19 rev 21:19And the foundations of the wall of the city,.... Which were twelve; see Rev 21:14 were garnished with all manner of precious stones: see Isa 54:11 not that there were all manner of precious stones in every foundation, but in them all there were, and each foundation had its stone peculiar to it, as follows; and which are not applicable to the persons of the apostles, who were not the foundations, but on them their names only were written: and besides, the order of them, as given in the Evangelists and Acts of the Apostles, is not certain, and always the same, to which these several stories may be adjusted; though they are doubtless comparable to them for their preciousness in the sight of God, and Christ, and all the saints; and for the brightness and purity of their doctrine and lives; and for their zeal in the cause of Christ; and for their solidity, and invincible courage and constancy: much less are these precious stones applicable to a set of ministers in the latter day, who are to be useful in the conversion of the Jews in the several parts of the world, where these stones are found, as Brightman thinks, but to Jesus Christ, the one and only foundation: and without entering into the particular virtues and excellencies of these stones, in general, they set forth the worth and preciousness of Christ, who is the pearl of great price; the pleasure and delight had in viewing his excellencies and perfections; his brightness, purity, and glory, and his firmness and durableness, as a foundation. There may be some allusion to the twelve stones in the breastplate of the high priest, on which the names of the tribes of Israel were written (i). The first foundation was jasper; it was laid with a jasper stone, of which see Rev 4:3. On this stone, in the breastplate, Benjamin's name was written. The second, sapphire; its colour is azure, or sky blue; it is transparent, and exceeding hard. Schroder says there are very good ones found in the borders of Bohemia and Silesia; but those which are brought from Pegu are most valuable: on this stone Issachar's name was engraven. The third, a chalcedony; it is of a misty grey colour, clouded with blue, yellow, or purple; the best is that which has a pale cast of blue; it is very much like the common agate; and the Hebrew word "cadcod", is rendered agate in Isa 54:12 though it is thought to answer to the carbuncle in the breastplate, on which was written the name of Levi. And Pliny (k) speaks of carbuncles, called "charcedonies", which R. Leo Mutinensis says (l) had their name in memory of the city of Carthagena. There is a precious stone mentioned in the Targum of Jonathan, called "cadcodin", which answers to the diamond in the breastplate, on which Zabulon's name was written; and in the Jerusalem Targum it is called "cadcedana", and answers to the emerald, which had Judah's name on it, and seems to be put for "chalcodin" and "chalcedana", which agrees with the name of this stone. The fourth, an emerald; of which see Rev 4:3. The best of this sort are the eastern ones. Schroder says (m) they are found in Cyprus, Britain, and other places, but not so good as the rest. On this stone Judah's name was written. (i) Vid. Targum Jon. & Jerus. in Exod. xxviii. 17. Targum in Cant. v. 14. Shemot Rabba, sect. 38. fol. 138. 2. Bemidbar Rabba, sect. 2. fol. 178. 3. (k) Nat. Hist. l. 37. c. 7. (l) Shilte Hagibborim, fol. 45. 4. (m) Pharmacopoeia, l. 3. c. 5. p. 18.”
Source
1832
A.D.
Adam Clarke Methodist
1762–1832
“The building of the wall of it was of jasper - The oriental jasper is exceedingly hard, and almost indestructible. Pillars made of this stone have lasted some thousands of years, and appear to have suffered scarcely any thing from the tooth of time. Pure gold, like unto clear glass - Does not this imply that the walls were made of some beautifully bright yellow stone, very highly polished? This description has been most injudiciously applied to heaven; and in some public discourses, for the comfort and edification of the pious, we hear of heaven with its golden walls, golden pavements, gates of pearl, etc., etc., not considering that nothing of this description was ever intended to be literally understood; and that gold and jewels can have no place in the spiritual and eternal world. But do not such descriptions as these tend to keep up a fondness for gold and ornaments? In symbols they are proper; but construed into realities, they are very improper. The ancient Jews teach that "when Jerusalem and the temple shall be built, they will be all of precious stones, and pearls, and sapphire, and with every species of jewels." - Sepher Rasiel Haggadol, fol. 24, 1. The same authors divide paradise into seven parts or houses; the third they describe thus: "The third house is built of gold and pure silver, and all kinds of jewels and pearls. It is very spacious, and in it all kinds of the good things, either in heaven or earth, are to be found. All kinds of precious things, perfumes, and spiritual virtues, are there planted. In the midst of it is the tree of life, the height of which is five hundred years; (i.e., it is equal in height to the journey which a man might perform in five hundred years), and under it dwell Abraham, Isaac, Jacob, the twelve patriarchs, and all that came out of Egypt, and died in the wilderness. Over these Moses and Aaron preside, and teach them the law," etc. - Yalcut Rubeni, fol. 13, 4. In the same tract, fol. 182, 1, we find these words: "Know that we have a tradition, that when the Messiah, with the collected captivity, shall come to the land of Israel, in that day the dead in Israel shall rise again; and in that day the fiery walls of the city of Jerusalem shall descend from heaven, and in that day the temple shall be builded of jewels and pearls."”
Source
1871
A.D.
1871
“the building--"the structure" [TREGELLES], Greek, "endomeesis." gold, like . . . clear glass--Ideal gold, transparent as no gold here is [ALFORD]. Excellencies will be combined in the heavenly city which now seem incompatible.”

Undated

date unknown
date
unknown
Glossa Ordinaria Medieval
“And it was. He had said that in this city there are walls and gates and foundations, and these among themselves, some greater, some lesser; here he sets forth what kind the wall is, and what kind the city itself and the foundations and the gates. The structure of its wall of jasper stone. That is, if anyone be instructed unto this, that he be a defender of others, he has this from the faith which he firmly holds and preaches, which the green jasper signifies, that he may be a defender of others. Gold. Gold denotes wisdom; but glass, the purity of faith, by which he promises with the mouth what he believes with the heart.”
Source
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.