The reference is to a precious stone. It is hard to determine precisely which of the precious stones known to gemologists today John may have meant with the term (the meaning of words can change over time). Of greater importance is the fact that the Holy Spirit guided John to use the same term as he earlier used to describe God: “he who sat [upon the throne] had the appearance of jasper and carnelian” (Revelation 4:3). John could do nothing better than use earthly comparisons—the term “jasper” was the most appropriate—to give a sense of what God looked like. John used the same term in Revelation 21:11 to relate that “the holy city Jerusalem” had “the glory of God,” and that glory looked like “radiance like a most rare jewel, like a jasper, clear as crystal.” This stone was the best earthly comparison that John could imagine to convey an accurate sense of the glorious God. In the present verse this divine comparative is captured in some way in the city’s wall.
18 The wall was built of jasper, while the city was pure gold, like clear glass.