This is the rock I'm using.
[INORGANIC:DEATH_STONE]
[USE_MATERIAL_TEMPLATE:STONE_TEMPLATE]
[STATE_NAME_ADJ:ALL_SOLID:death_stone][DISPLAY_COLOR:0:7:1][TILE:'*']
[SPEC_HEAT:250]
[IGNITE_POINT:10020]
[MELTING_POINT:10016]
[BOILING_POINT:10017]
[SOLID_DENSITY:10000]
[IS_STONE]
[STATE_COLOR:GAS:ASH_GRAY]
[STATE_NAME:GAS:death dust]
[STATE_NAME:GAS:death dust]
[SYNDROME]
[SYN_NAME:death dust]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_INHALED][SYN_CONTACT]
[CE_NECROSIS:SEV:1000:PROB:100:RESISTABLE:START:1000:PEAK:1000:END:100000]
[CE_PARALYSIS:SEV:1000:PROB:100:RESISTABLE:START:1000:PEAK:1000:END:100000]
[CE_UNCONSCIOUSNESS:SEV:1000:PROB:100:RESISTABLE:START:1000:PEAK:1000:END:100000]
It's a subterranean rock ONLY if it goes on the surface (if it isn't below 10015 Dorf Degrees) it'll boil. I accidently deleted the save that I did some changes on, so if this doesn't work I'll have to change it again.
But more or less it worked this way...
========== <- hatches with the stones stored on top of them.
OOOOOOOOO <- empty space, because the fire would heat the stones if it were this close.
__B__B__B__ <- path with bonfires in it. Typically three wide. I haven't perfected it yet. I have a few ways I do it, When I add the entrance to my current fort, I'll upload it so you can see it.
[][][][][][][][ <- solid ground
Hope that little visual turns out alright for you and you understand it.
But more or less you pull the lever hooked to the hatches, the stones drop onto the bonfires, the boil and poison the air. Note that you can have multiple z-levels of hatches just in case or store multiple stones on the hatches so you hit a more solid area. You are relying on the stuff to be inhaled and/or contact them.
I could mod the bonfires and stones so it could have an outdoor use, but underground temp is steady.