Marble or Granite shelfs in a hifi rack?

If it were b/ween the two, I'd rule out marble & try granite instead. Particularly for a suspended wood floor. BUT, things are as Twl explained & all other have illustrated, i.e. which frequencies are you acting upon & what compromises are you ready to make (i.e. acting in one direction can bring along unwanted effects from a totally different direction).

Among others here, Jadem6 & Redkiwi have spent a LOT of time trying all available materials under the sun under their components :)! Maybe they can add an idea or two... Cheers

Granite, marble, glass, maple, all have a sound that some may like for reasons other than being accurate. MDF is a as bad as anything I have used. I have used all of these that I have mentioned and many more. Neuance and Sistrum made a joke of all the materials i have used in the past. They make my system sound like music. They may not be perfect but compared to all the other materials I have used I find much less colorations in sound.

Whats Neuance and Sistrum?

Well Brulee, I may not have yet made the plunge to begin seriously investigating alternatives, but I can agree with you about MDF. I suppose there may be worse materials one could employ, but on the whole I think the entire MDF paradigm is basically a scam rack makers (and retailers like Audio Advisor) have attempted to perpetrate on audiophiles in order to get away with using an easy and cheap shelf material.

Ulf, Neaunce is some type of composite shelf that absorbs vibration from floor and the component. If you dfo a search there was a long thread a month or two ago.