Here is what I believe.
In theory the best shelf will achieve two, mutually exclusive things. First that it is light and rigid. Second that it is damped.
Light and rigid is vital to ensure vibration energy is dissipated quickly. If you use a heavy shelf then energy will be stored and released slowly thereby smearing the music in the time domain. Some people like this effect since it results in a weightier bass, albeit less articulate and with incorrect pitch. If you use a floppy shelf you will get a similar effect, but that will be more focussed on a single frequency than a more rigid shelf. The "easy to come by" light and rigid shelves, such as glass are still fairly heavy, and they tend to ring badly.
Damped is important to avoid the release of energy being focussed on one frequency range. This is the problem when you use stuff like glass, granite, acrylic and corian. Corian is the best of these but smears the bass and still has a resonant peak. Those shelves that are not well damped tend to need to sit on bumpers rather than spikes - simply to avoid the resonances in the metal rack setting off resonances in the shelf. But the problem is that in not using spikes you will get more smearing of the sound due to energy storge in the rack.
So the ideal is a very rigidly welded steel rack (check that welding is more than one tack weld per join), spiked onto the floor; spikes screwed into the rack, supporting the shelf. The rack would ideally be put together with non-uniform shape - which is the principle that Sistrum focusses on. The shelf would be an ideal blend of light, rigid and damped - which is the ideal that Neuance aims for, and does a pretty good job of.
There are many shelf products and isolation devices that go in a different direction altogether. For example there are the Symposium style heavy and heavily damped products, which aim for neutrality and black background, but give up on speed. If your musical values include PRAT, then do not go this path. By the way I reckon a lack of understanding of PRAT is the principle reason for boring music systems. Many audiophiles listen for impact, detail and neutrality, but find it hard to listen for PRAT. PRAT is all about whether or not your system can communicate the rhythm and swing in the music and tends to require minimum smearing of transients, which tends to be most damaged by heavy support shelves and racks. The reference to Maple above is a decent example of this - the sound is reasonably neutral, but the mass means PRAT is badly compromised. I think PRAT is overlooked because in our straining to hear differences between stuff, we focus on the most easily discerned differences which are tonal colorations, impact and detail.
There are also many footer products that try to compensate for a bad rack. The hard footers are OK but are not very neutral and only have a significantly beneficial effect when you have put too much mass in your shelf/rack. The soft footers suffer the same problem as the 'less than rigid' shelf - they channel energy into a frequency range. Many of these claim, as do the bladder products, to channel the energy into very low frequencies that do not affect the sound. They do not affect neutrality, or detail, but they generally sound 'swimmy' and indistinct - in my opinion because they allow lateral movement, which is the worst form of energy for stereo equipment in my experience.
So the simple message is that Granite is way too expensive for its performance as a shelf, and I recommend you look elsewhere.
