I've given up for a time, so I currently am doing something totally irrational - generating worlds with duplicates of raw files.
So far, my embark is at the very edge of an ocean, with beaches made from solid native silver. The soil layers are frozen disgusting goop that falls from the sky in evil biomes, and it even drops stones. So far, no unforeseen things have happened. Besides the whole "Native silver beach" thing.
Ha! Once I installed Genesis (a stand-alone mod) in with regular DF to see what would happen. Every soil tile began a rapidly altering between every material in the game. Giant bat liver tissue shoes. Civs of dragons. Total chaos.