A.T. Robertson Commentary Revelation 21:18

"And the building of the wall thereof was jasper: and the city was pure gold, like unto pure glass." — Revelation 21:18 (ASV)

The building of the wall (η ενδωμησις του τειχους). Or ενδομησις, elsewhere so far only in Josephus (Ant. XV. 9. 6, a mole or breakwater) and in an inscription (Syll. 583 31), apparently from ενδομεω, to build in, and so the fact of building in. The wall had jasper (verse 11) built into it.

Was pure gold (χρυσιον καθαρον). No copula ην (was) expressed. The city shone like a mass of gold in contrast with the jasper lustre of the wall.

Pure glass (υαλω καθαρω). Associative instrumental case after ομοιον. Hυαλος (apparently from υε, it rains, and so raindrop) in N.T. only Re 21:18,21.