I have had excellent results with 1/2" glass (I use it on a Target rack for extra isolation) with vibrapods between the glass and the normal Target shelf as well as between the glass and component as Sdcampbell says. In this configuration (a vibrapod sandwich), much better than granite, marble or more mdf, all of which I've expermimented with. If you are supporting a bit of weight, I'd suggest the 1/2"-it is very dead-little or no "ringing" or tilted up high frequencies (at least to my ear) which you will get with granite or marble, and none of the muddiness that you get with MDF. If you have lighter components and want to save some money, you can actually buy security glass or "laminated" glass, which is much thinner, but consists of two layers of glass sandwiching a glue like material to prevent shattering. This in turn, really serves to damp out vibrations and eliminate ringing that is inherent in thinner sheets of glass. Laminated glass is also going to be a fair bit cheaper than 1/2". I use the laminated as well, also with good results. Again, as Sdcampbell said, use the vibrapods in conjunction with this.
