Materials:

Materials were purchased at Lowe's and/or Home Depot.

3/4" polystyrene sheets, Package of 6 large pieces  = about $5

tube of "liquid nails" brand adhesive. (General construction or All purpose type) about $3

Tile Grout - powdered with sand, your choice of color. Varies in price, from $5 - $25.

Grout Sealer - water based, varies in price, from $8 - $35

Paint Brush about $2

Knife, Razor Blade, Exacto Knife or another cutting tool of your choice.

Optional Materials:

 sand (large bag of play sand = $3 - $4) & Great Stuff brand foam insulator (about $4)

Construction Steps:

1) Cut out desired shapes from polystyrene. You can make smooth edges or rough edges (just break the polystyrene instead of cutting). You can also file the edges to give a more realistic profile to the "rock". You can also use Great Stuff foam insulator ( "foam in a can" ) to add a different shape to your rock. Follow the directions on the can!

2) Use the liquid nail to attach the pieces of polystyrene together in the desired pattern. Use the adhesive sparingly, a little goes a long way! Allow the pieces to "set", give it about 15 or more minutes before going on to the next step.

3) Mix up the grout per instructions on the bag/box. A little side note: I use the cheap gray grout that is about $10 for 50lbs from Home Depot. It looks great & lasts forever! Make the first layer a bit watery. Paint the grout onto your foam, making sure you get all cracks and crevices.

4) Allow the 1st layer to dry (up to 24 hours). Mix up another batch of grout mix, except this time it should be thicker, somewhere between a syrup and peanut butter, depending on your preference. You may also add playsand to this layer to give it a more realistic and rough finish. Paint or slap on the grout onto your foam. Allow this layer to dry for 48-72 hours. It may take even longer depending on the thickness and other variables.

5) After you have allowed the "rock" to fully dry, you can apply the grout sealer. This step is important, the grout will flake off if you don't seal it! Follow the directions of the sealant you are using. Usually you will just "paint" a couple of layers onto your "rock" and allow it to cure for 24 or more hours.

I used this method to make rocks for my leopard geckos. I have had no issues or problems with these in my cages. However, use this method at your own risk, I do not guarantee the durability or safety of this project/product!