Albert Barnes on Revelation 21:19

And the foundations of the wall of the city. Rev 21:14.

Were garnished. 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. Barnes on "Re 21:14".

Was jasper. Rev 4:3.

The second, sapphire. This stone is not elsewhere mentioned in the New Testament. It is a precious stone next in hardness to the diamond, usually of an azure or sky-blue colour, 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 colour, and of a lustre nearly like wax. It is found covering the sides of cavities, and is a deposit from filtrated silicious 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 colour. The name chalcedony is from Chalcedon, a town in Asia Minor, opposite to Byzantium, or Constantinople, where this stone was probably first known.-- Webster's Dic.

The fourth, an emerald. Rev 4:3. The emerald is green.

(a) "foundations of the wall" Isa 54:11