I did make the land parts by a combination of glass, and foam / tile grout / epoxy. Not really natural materials, but with a natural looking result. If you'd insist on really natural materials, you could use large pieces of stone / wood and peat or even clay. The main problem would be here that the materials would be natural, but the look most of the time won't - simple because natural banks and shores are not easily rebuilt in a tank.
tried it a long time ago with a 20 long and natural topsoil/clay mounds under gravel. ill tell you from my experience... that method doesn't work.... hahaah. Maybe something like volcanic rock would hold its shape long term