In this world, the inhabitants are obsessed with aesthetics and themes.
There are two especially notable cities. One city values reflections and the other city values transparency.
Unknown ages ago, the cities were founded by twin sisters.
One sister discovered a piece of reflective obsidian and was enthralled by her image. She gathered a group of artists and craftsmen to design mirrors, so that she may gaze upon herself.The other sister discovered a quartz crystal. This sister was fascinated by its clarity and refractive properties. She, also, gathered interested people to craft wondrous, transparent objects.
As the two cities flourished, new materials were discovered that provided the same reflective and refractive properties. The citizens coveted and gathered these materials to glorify their two cities.
Select groups of the two cities have joined to form guilds. While their alliances are not illegal, they are socially discouraged. Despite community coercion, these guilds have produced devices like telescopes, microscopes, and lighthouses.
Obsidian was soon replaced by polished bronze. As their comprehension of chemistry evolved, mirrors were fashioned of glass coated by tin, mercury, or silver.
The refractive city has, also, utilized glass to created lenses and prisms. Buildings constructed of ice have become a current trend, but only the most pristine ice is considered fashionable. And, it has a grand aquarium of x-ray fish, jellyfish, glass catfish, and icefish.
These cities have covered every surface in, and constructed every object of, materials that have their valued qualities. When the sunlight strikes the twin cities, their glare has been known to blind travelers. The citizens have learned several methods to navigate their respective cities. They wear headwear to shade their eyes; they have accustomed themselves to the bizarre, misleading images; and, they carry parasols.
Many young artists reject the idea of colorless crystal and achromic speculums. They want some color! Mainstream artisans chide them for their foolish fads. They feel the amateur creations have gaudy hues and... opacity! The avante-garde have formed Iridos, a community of rejected artists who categorize everything by colors.
Some antiquarians mourn the advent of glass. They feel that it dilutes the cities's themes. They speculate that the architects no longer enjoy the reflective and refractive qualities, but the glass itself. The antiquarians hoard ancient obsidian and quartz relics to remind themselves of the "glory days of yore" and the cities's "true" themes.
 Religion and Magic
The religions of these two cities have been affected by their thematic qualities, as well. The transparent city worships a wind god because they percieve wind as a perfectly transparent force. The reflective city worships a moon goddess because they believe the moon is a large mirror.
They both agree that water is a wonderful element, but, obviously, for different reasons. The reflective sect believes that water symbolizes mental reflection of oneself; it provides a medium to reveal truth. The transparency sect states that reflections allow one to indulge in vanity and superficiality. The transparency sect believes that water symbolizes clarity and purity of thought; it allows one to see the internal truths. The reflective sect states that clarity is only effective when the truths are depthless and shallow.
Some sorcerors and theologians claim that reflective surfaces are gates to a mirrorverse of strange spirits, ghosts, and dopplegangers. Prophets of the reflective city claim that clairsentience is like peering into a hazy mirror that reflects the future into the past. Prophets of the refractive city claim that clairsentience is like peering into a murky prism that holds a hidden possibility.
Original RPG.net thread: http://forum.rpg.net/showthread.php?t=377740