In the future, humanity discovered that there are twelve points of energy distributed equally around the globe, in opposite to Ivan T. Sanderson's Vile Vortices. People who live at these twelve points experience greater connectivity with their fellow beings, a greater sense of satisfaction with life, greater ability to achieve their goals, better health, etc. Crime is practically nonexistent in these cities. Subsequently, megacities were built at these twelve locations. Inspired by the twelve gems of the Hebrew's priestly breastplate (ḥōšen), the twelve gems of the foundation of the new Jerusalem in Christian Book of Revelation, and the twelve birthstones, humanity built each of these cities out of a particular gemstone. Since the identities of the ancient stones is a matter of debate, the modern birthstones were used.
There were also several gemstones that were proposed, but ultimately rejected:
And, just for fun, here are some outputs from the AI that weren't what I wanted, but that I still thought were worth keeping:
These illustrations were drawn using Stable Diffusion 2.1.