Actually the ctype is 2. Stone is the second element in the game.
I was experimenting with the results from my recent save (ID:2059011) by mixing the DSTW(FIRE) in various containers with stuff under different conditions, when I noticed that the CO2 becomes CTYPE:STNE. Also you may want to see ID:1876734 which I made a while ago. May be related.
seems to cause it to "rain" bubw