Not sure if this is the look you are going for or not, but try styrofoam and acetate.
Years ago as a kid I played around with making underground scenes using white styrofoam, and bottles of white-out(much to my parents dismay, and my own grounding)
Now I use Acetone, or nail polish remover for a similar effect. Cut you styrofoam blocks to the overall shape you want, making the intial piece larger than where you want the finished piece to end. Then give a very very light coat of acetone, brushed over it.
The styrofoam, dissolves, leaving behind a very flowstone looking end result, without a whole lots of time needed.
Caveats
1. w/ the acetone a little goes a long way. Too much and it will eat away all the foam.
2. Make certain to do this in a very well ventilated area.
My two cents
hm, I have used that method before actually, tough I used revel glue for it and used it to make craters in asteroids. Oh and if you let a thin slip of foam get eaten entirely the result can be used as an "energy blade" of sorts. But as you say this should really be done in a well ventilated area, as the fumes are horrible.
