I just discovered that one of my modding attempts had some surprisingly fortunate, but weird and undesired effects.
I wanted to remove all of the useless rocks, like orthoclase and microcline, from the game in the hope of getting more gems and metal ores instead of large clusters of crap. My attempt involved just taking the matgloss_stone_mineral.txt entries and removing the brackets around all of the useless stones' names, believing that this would remove them from the game without actually deleting all of them.
Well, then I started up a map, used Reveal to survey the landscape, and found that there were mica and alunite clusters amongst my various igneous layers. This annoyed me, but I decided to make a fort anyway and just deal with their presence. A couple ingame years later, I chance to look at a mica stone. In its uses list, it claimed that it was an ore of aluminum, and was worth 120 instead of the expected 3. I found this more than a little confusing, so I looked at my stones screen.
Sure enough, mica is now, inexplicably, an ore of aluminum. Native aluminum apparently doesn't exist, despite the fact that I didn't mod it at all. Horn silver also doesn't exist, nor does calcite; instead, brimstone and alunite take their places, uses, and values. This was not what I had intended when I tried to remove useless stones from the game...
On the bright side, I now have flux (calcite-called-alunite) on a purely igneous map, as well as... Oh, probably about 400 native-aluminum-as-mica so far, with at least 2 more totally-untouched large clusters. Once I get a blacksmith up to legendary, I think I'm going to make enough aluminum tables and chairs to furnish my entire public dining hall, and maybe make all of my nobles full-aluminum rooms (the king'll get adamantine, since I've got a funhouse on my map).