I replaced all my rack shelves with 3" maple pieces and have this Adona piece which is part granite and some polymer backing.
Lists for $140. Sell for $95 + shipping as it is heavy but can be easily boxed.
From their website:
Our famous three-element platforms combine the advantages of several different material technologies. The genuine granite top layer provides stunning beauty, a diamond-hard surface and high mass while the inner conduction layer operates as a band-pass filter, as it transfers energy to the lower damping layer.
The result is a sonically-neutral platform that will enhance clarity and detail, without any of the harshness usually associated with the use of crystalline materials. You'll experience a sense of realism never before obtained with an audio platform. No more muddy-sounding bass guitar runs or cymbal hits that sound like air jets. And, the sound-stage will blow you away!
With thousands of units in the field, this time-less design has remained virtually unchanged since December of 2001. Our competition has yet to develop a platform that performs as well as this simple design, at any price or with any degree of complexity. Adona platforms offer more "natural-sounding" performance than carbon/composite, acrylic or HPL-coated MDF designs and are far less "resonant" than solid maple, solid-granite, glass or metal
