Marble or Granite shelfs in a hifi rack?

It depends upon which frequencies you wish to isolate. Different materials have different characteristics. I know that marble cracks very easily, and that could be a problem. Granite "rings" if it is not real thick. Actually, a very dense wood like Cocobolo is better than either of those stone products.

Mknowles, I mentioned that Granite has that ringing. I didn't say that marble does. I think that marble, being a sedimentary rock, has more in common with limestone, than with granite. Therefore, I would guess that marble would not ring much, or at all. However, it does break very easily, if it is not supported fully from the bottom. As such, I would not expect it to be suitable for shelving on an audio rack.

As far as granite ringing, I know that 3/8" thick definitely does, and I haven't done specific tests on other thicknesses. I have read some material that referred to granite TT bases causing unwanted resonances.