Revelation 21:21 (ESV)

21 And the twelve gates were twelve pearls, each of the gates made of a single pearl, and the street of the city was pure gold, like transparent glass.

Pearls are pebble-sized “stones” made of carbonate, naturally formed inside the shells of mollusks (a sea-creature found, for example, in the Mediterranean). Due to the challenge of finding and harvesting pearls, they were rare—and hence expensive—in Bible times (Job 28:18; Matthew 13:45–46). The prostitute of Revelation 17:1–18 arrayed herself in great riches, including pearls (Revelation 17:4; Revelation 18:16).