What is the perferred material for a TT shelf?

If it were me I'd use the roller bearing assembly type Isolators in conjunction with, I.e., on top of the sand filled boxes.  Should be a match made in Heaven given the sandbox is acting primarily in the vertical direction and the roller bearing assemblies act primarily in the horizontal and rotational directions.  

I like the direction williewonka takes but would experiment with the final grounding to the granite. His method does allow for easy checking on the isolation of the granite from vibration--just sprinkle salt or sugar on the granite and play your music. Cheers.. to the eyes...

I use to think granite was a good material, then switched to thick Maple blocks.  They sound much more natural, and still has the dynamics, and speed that granite provides.  So yes Maple is currently still my favorite material.  Obviously what ever your equipment sits on it takes on that character.  I have tried many different materials, carbon fiber, concrete, granite etc.. But wood just sound more natural, less harsh, yet still has good dynamics, speed, but just with a much more natural sound.  I would love to try other woods but the time and money to try them all lol