Albert Barnes Commentary Revelation 21:19

"The foundations of the wall of the city were adorned with all manner of precious stones. The first foundation was jasper; the second, sapphire; the third, chalcedony; the fourth, emerald;" — Revelation 21:19 (ASV)

And the foundations of the wall of the city. (See Barnes on Revelation 21:14).

Were garnished. They were adorned or decorated. That is, the foundations were composed of precious stones, giving them this highly ornamented and brilliant appearance.

The first foundation. The first row, layer, or course. (See Barnes on Revelation 21:14).

Was jasper. (See Barnes on Revelation 4:3).

The second, sapphire. This stone is not mentioned elsewhere in the New Testament. It is a precious stone, next in hardness to the diamond, usually of an azure or sky-blue color, but of various shades.

The third, a chalcedony. This word occurs nowhere else in the New Testament. The stone referred to is an uncrystallized translucent variety of quartz, having a whitish color, and a luster nearly like wax. It is found covering the sides of cavities and is a deposit from filtered siliceous waters.

When it is arranged in stripes, it constitutes agate; and if the stripes are horizontal, it is the onyx. The modern carnelian is a variety of this. The carnelian is of a deep flesh-red or reddish-white color. The name chalcedony is from Chalcedon, a town in Asia Minor, opposite Byzantium (or Constantinople), where this stone was probably first known—Webster's Dic.

The fourth, an emerald. (See Barnes on Revelation 4:3).

The emerald is green.