Revelation 21:18 New English Translation

¹⁸The city’s^[a] wall is made^[b] of jasper and the city is pure gold, like transparent glass.^[c]

Revelation 21:18 tn Grk “and its wall”; the referent of the pronoun (the city) has been specified in the translation for clarity. Here καί (kai) has not been translated because of differences between Greek and English style.
Revelation 21:18 tn The phrase ἡ ἐνδώμησις τοῦ τείχους (hē endōmēsis tou teichous) is difficult to translate precisely. BDAG 334 s.v. ἐνδώμησις states, “primary mng. ‘interior structure’; in our lit. prob.=construction, hence material τοῦ τείχους Rv 21:18.” The phrase could then be translated, “the foundation of the city wall was jasper” or “the material used for the wall of the city was jasper.” The latter alternative has been used in the translation because the text goes on to discuss the foundation in 21:19 (using the term θεμέλιος [themelios]), which is somewhat redundant if the foundation is mentioned here.
Revelation 21:18 tn Or “transparent crystal.” See L&N 6.222, which notes the emphasis is on transparency here. The same Greek word, καθαρός (katharos), means both “pure” (referring to the gold) and “transparent” (referring to the glass).

New English Translation (NET)