Granite surface plate

Ass soon as you get it, bend down to 3" tall.

If you look at the range of hardnesses in turntable materials, granite is probabably harder and denser than any of them. In other words, its mechanical impedance (resistance to being set in motion by external vibration) is high. This is not what you want for a turntable base, as it will take the vibrations transfered from the turntable and bounce them back into the turntable to be picked up by the cartridge, effectively raising the noise floor of the extracted signal. A turntable base should absorb and dissipate the turntable vibrations.

I've gotten much better results from a butcher block style maple cutting board. Mine is 3.5" thick and audibly lowered the noise floor when I placed it under my turntable. You can spend a lot of money on a butcher block cutting board or save a bunch by getting it from Overstock.com. It will likely be much less than a finished solid maple board, and the butcher block construction breaks up resonances.

MIne is just like this one.

A high-mass paving stone sitting on top of an under-inflated bicycle tire is another old idea (first proposed by Frank Van Alstine in the early 80's).

I did exactly what you propose. 3" granite surface plate on a machine stand from a company called Production Tool. Works fantastic with my VPI HW-19. Grizzly sells them nationwide. You don't have much to lose.
Try this and the next page.
http://www.grizzly.com/search/search?q=granite%20surface%20plate&rankBy=relevancy:descending&page=1

If you are serious about a turntable platform you will acquire an electron microscope base. Most go for about $10K. Mine works well.