HI ALL.

If it is a stone effect you are looking to recreate, as in the stone of underneath a waterfall, then mixing sand into the resin might give you a rough texture you want.  You would also be likely to need a pigment to help the base colour.

Using the foam as a base can be a good idea.  As it won't be seen, if you didn't want a foam backing, you could easily cut it out afterwards.  If you do that, I would recommend using a layer of Woven Glass Fabric to strengthen and reinforce the object.

Another thing to check is that any resin you use is safe for use with pets as they may knaw at it or chew it. None of our resins are certified food safe (or equivalent) so it might be best consulting with a pet specialist to be safe.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

to add something to what warren said, make sure you use dry sand if you do so. 

Thanks  guys. I had planned to put in a couple of layers of glass to help with the strength. Also is there anything that polyurethane foam doesn't stick to so I can mix a bit at a time to make different shapes?

Give Bentley Advanced Materials a ring http://www.benam.co.uk/ they do materials suitable for making your own aquarium features so may be the stuff you're looking for (they are in Kidderminster)