I got all hyped up reading about Oil of vitriol - the medieval name for sulphuric acid - which is used in gold- and silversmithing.
It can be made by pyrolysis (dry distillation - like charcoal) of vitriols - certain metallic salts that can occur in old mines for example.
So I made this blue vitriol graphic that was supposed to spawn very rarely inside Urw caves.
The thought was then to process it to oil of vitriol which would be fundamental in allowing to craft precious metals.
But - I couldn't mod it to appear (only) inside caves. So I decided to scrap the idea for now. Here's the graphic I already made for blue vitriol - just in case someone gets a spark for making semiprecious stone mod or smth. I was thinking about it but lacking this mysterious poison my enthusiasm vaned. Maybe later. Need to do more reseach.
Here's the code I got to work. ("Cave" and "Cave_floor" don't seem to work)
.Blue vitriol. (4) [root]
[NAME:Blue vitriol]
[TILEGFX:bc-vitriolb]
[SIZE:S]
[SPROUT:1]
[MATURE:1]
[WITHER:12]
[REGION:northern southern western eastern]
[TERRAIN:mountain]
[SYMBIOSIS:rocky]
[POPULATION:1]
[FREQUENCY:3]
[COMMONNESS:70]
[EFFECT_RAW:poisonous]
[CARB:0]
[FAT:0]
[PROTEIN:0]
[HERB_KNOWN:all]
-end