When I showed with Golden Sound at CES I had the local Las Vegas stone place cut a bunch of black Italian marble slabs for use in my Promethean mass on spring iso stands, two slabs per iso stand for total of about 8 marble slabs. Marble and granite and bluestone work great in mass on spring installations due to their inherent mass and stiffness. Any tendency of these materials to ring is minimized due to isolation of the top plate as well as the component on the top plate. Also, damping can be applied to the top plate to address any residual vibration. The only downside was shipping costs of the marble slabs back East. 😛