If you go mass-loading marble, no point in getting small pieces.
My base is 60inches by 18 deep and 5 high standing on 8 solid cones on a concrete slab floor. The player stands on a stonework plinth on top of that. Weight in all must be around 1000 pounds. Standing on the stone slab makes the system effectively infinitely mass-loaded limited only to the mass of planet Earth. The stylus/disc interface cannot be affected by any outside vibration or sound waves. CD/SACD player, phono and pre-amps all stand on the slab.