Polyurethane is available in a FOAMING formulation which when the two components are mixed, creates a self-rising foam which can and does come in a variety of hardnesses (depends upon the company you get it from).  BJB is one of the major suppliers in the US for urethane foams, although Smooth-ON has their version, Zeller, Inc., Burman's, Monster Makers carries waterblown foams, as does FX Warehouse in Orlando or Polytek.  There are numerous places you can get it.  Urethane foams - some of them - are VERY CAUSTIC - namely they create a gas when foaming which is carcinogenic and you can literally DIE if you inhale the fumes.  They say if you're doing (some)versions of urethane foam, you should wear a respirator, do it outside, get ready to mix and pour, HOLD YOUR BREATH, POUR IT and then run like hell the other direction UPWIND of the stuff.  VERY much a scary product.  There are other foams which are self skinning that come in a variety of SHORE HARDNESSES (rigid foams) and there are some that don't skin over but have a softer, more pliable consistency about them like foam rubber (but not rubber)...more like sponginess but not all that flexible.

THE BIG problem is how do you create anything from these pourable, self-skinning, flexible foams??? Key is that you must have a negative mold of whatever you plan to make.  So if it's ROCKS you want to do, then you should cast some REAL rocks in a good, flexible (like GI-1000 from Silicone, Inc.) silicone negative molds. THEN (with proper release agent) you can cast urethane foam rocks in those molds and the negative mold being silicone - very flexible - can be removed and used again and again....many times before you start to loose surface detail.

There is a company that makes a urethane product (can't remember the name now) that model makers use which CAN be gotten in fairly large blocks if I'm not wrong (or many sizes large or small).  The stuff is CARVABLE, can do it with a pocket knife and a Sur-form tool even, very rigid and takes EVERY nick and scratch you put on it but it CAN be sealed with a fiberglass resin when you have your rocks created or whatever.  There is also a BLOWN product which is what these theme parks create stones and rocks out of but it's a cement type product though lightweight, sturdy and is a form of urethane but not.

Urethane foam is an interesting product and with the self skinning stuff you can get some cool props made though it's not too inexpensive.  And what you wind up with is styrofoam like rocks but harder than styrofoam.  BUT it still can be dented, scratched or nicked - but it is effective stuff for a VARIETY of things.  If you want a comparison to what it will be like - get a can of spray expanding foam insulation filler from the hardware store and it's about the same kind of thing.  Give you an idea of what it will do.  Urethane foam however, uses an A and B component, mixed 1:1 and then poured into a mold which will then fill up by expansion of the foam.

There is more to making this stuff than meets the eye however as you need a solid technique for mold making in silicone or urethane molding material which is LIKE silicone but cheaper, and then you can mold urethane foam in that.  And it also means you have to have a rigid mother mold around the flexible urethane or silicone negative of whatever you're casting too.  So it's not a cakewalk but learnable by a qualified tech.

Tony Di