Hyalos (G5194) refers to glass or crystal — transparent, pure material. It appears only in Revelation (4:6; 15:2; 21:18, 21) to describe the sea before God's throne, the heavenly sea mingled with fire, and the foundations and streets of the New Jerusalem.
John's use of hyalos in Revelation clusters around the divine throne and the New Jerusalem — creating an environment of translucency and purity. The sea of glass 'like crystal' before God's throne (Rev 4:6) recalls Ezekiel's vision of an 'expanse, shining like awe-inspiring crystal' (Ezek 1:22). In Rev 21:21, the streets of the New Jerusalem are 'pure gold, like transparent glass (hyalos katharon)' — the ultimate paradox: impenetrable substance made transparent by purity. In the old creation, sin made things opaque — we 'see through a glass darkly' (1 Cor 13:12, KJV — the word there is esoptron). In the new creation, everything is transparent because nothing is hidden. The hyalos city has nothing to conceal.