Perhaps you could fill the relief holes with the planet Earth itself, then skip around the galaxies, spreading the message of rock across the universe.
If you want really heavy, just use a teaspoon of neutron star. Or depleted uranium, heavy enough that they make anti-tank rounds out of it. Or just get some old fasioned cast iron weights and mill them down to size. Or fill them with the souls of the ****ed. Or the souls of the saved, your choice.