This fully depends on your sub strait that you will be bonding your tiles to.
I would not recommend using any riverstones due to how difficult it will be to get a good bond.
This would be a good sub strait for real tile. It may work with river stone. Best part is the light weight. It is just rigid high density Styrofoam with fiberglass cloth on both sides for bonding and strength.
Back cutting the real river stone on the backside with a diamond wheel in a 4 inch angle grinder will increase the bonding strength. This will also give you a chance to keep the height of the stones consistent.