Gold gets its colour (commonly shades ranging from yellow to orange) largely from the impurities in it. Pure gold (with no alloy) is so weak (and hence malleable) that it can easily be hammered into the thinnest of leaves—and so even become translucent. The comparative “like clear glass” communicates the purity of the God who presence fills this city (see further Revelation 21:22–23).
18 The wall was built of jasper, while the city was pure gold, like clear glass.