I saw a reference to a dungeon called "Cobweb Caverns". Don't know anything about it; it's probably just a dungeon with cobwebs as a prominent feature... but my immediate image was a dungeon made of cobwebs instead of stone, perhaps a sprawling labyrinth woven by giant trapdoor spiders. It would be suspended high in the trees, so there would be quite an adventure just reaching it.
That got me thinking about other unusual materials to make a dungeon out of, and how these materials would affect the way the dungeon played out.
A dungeon made of sturdy but mostly transparent glass bricks would sort of be like a habitrail, with light and occasionally shadowy outlines visible for short distances through the walls. Monsters could be warned of adventurers before they've actually reached the same section of the dungeon, and adventurers might spot a treasure in the next room, but not be aware of how to reach it.
A dungeon made of thorns would offer a minor hazard if adventurers run or are pushed into a wall, and a major hazard if it catches fire. There may be glints of light visible in adjacent rooms, but not the same degree of visibility as a glass dungeon. Perhaps the thorn dungeon encircles the lost palace of a sleeping princess...
Any other interesting ideas for dungeon materials?