Buying stone at a quarry can be expensive. Instead, you can choose to make your own artificial rock using Portland cement, peat moss and sand or perlite. This is a recipe for hypertufa. Hypertufa is a substitute for tufa, a porous limestone that was once popular for alpine garden planters. As such planters became scarce, hypertufa was invented as a replacement. Hypertufa has the advantage of being inexpensive to make and easy to sculpt. If you can make a mud pie, you can sculpt hypertufa rocks.
Put on protective gear when you work with Portland cement. This includes breathing protection and goggles to protect your eyes and lungs, as well as long sleeves, long pants and gloves.
Mix 3 parts Portland cement with 4 parts peat moss and 5 parts perlite in a 5-gallon plastic bucket.
Add water and mix with a stirring rod until the contents are the consistency of a mud pie.
Create a mold for your stone by arranging metal flashing into a circular shape. Place flashing on the ground and reinforce the sides by driving stakes into the ground up against the flashing with a rubber mallet.
Pour the hypertufa mix into the flashing. Cover with plastic wrap.
Remove the metal flashing from the hypertufa stone after 24 hours. Round off the edges of the hypertufa stone with a wire brush. Recover it with plastic wrap and allow it to continue curing and hardening.
Remove the plastic wrap every day and mist the stones with water from a spray bottle. Cover with the plastic wrap again. Continue this until the hypertufa has completely cured.