What role does “gold” have in this paragraph?

The term gold bookends the present paragraph—here with a reference to the golden measuring rod and in Revelation 21:21 with a reference to the city’s golden streets (Revelation 21:21). Between these two references is one more reference to pure gold (Revelation 21:18), presumably referring to the city’s houses. Gold was the most expensive metal known to the ancient world, and so in the book of Revelation (and elsewhere in the Bible) it is used to convey heavenly glory (Revelation 1:13; Revelation 4:4; Revelation 5:8, etc.). The prevalence of gold in this scene points to the heavenly splendour of the city’s Bridegroom. That even the angel’s measuring rod is gold points to the glory of his Sender.

That the vision is symbolic as opposed to literal (as indeed so much of the book of Revelation) is illustrated by the fact that a measuring rod of gold of sufficient length to measure a city of this size is a literal impossibility. Gold, after all, is a very weak metal so that a golden rod of any substantial length would bend to such a degree that you could not lift its distant end off the ground. This comment pertains also to the reference in Revelation 21:18 to the city itself being of pure gold.