What is the perferred material for a TT shelf?

I'm designing a shelf for my turntable. Any recommendations as to what material to use. 

I went to my local acrylic workshop in an industrial area and had them custom make mine. It consists of two 1" slabs of clear acrylic. The top one has a channel routed out to the shape of an acrylic cover I had made so when it sits on top, it can't slide around. I used isolation dots from herbie's audio lab under the base layer and between the two slabs.  The look is beautiful and the functionality great. You can see it on my virtual system page.
Depends on the type of suspension you have.  If it is the now typical solid (or mildly isolated) suspension then hard maple butcher block makes an excellent base.  Other woods may work too, but hard maple is the most recommended.  Using harder materials like stone, glass, or maybe acrylic provides less damping and can negatively impact the sound.  There are of course folks who will insist the harder materials work fine, but you takes your chances....  Now if your turntable has a spring suspension you may have more choices, the springs help isolate the table from the shelf material, though I would still suggest maple. 
sorry for the misspelling. If I go solid wood, does it really matter what wood: maple, cherry or walnut? 
My current solid and my previous suspended table both benefited from being spiked to a 2" thick maple slab isolated (with those ubiquitous) cork/felt constructs from a shelf of baltic birch plywood  screwed down tight to a pair of wall hung cantilevers. 

Improved texture and tuneful bass. The maple acts as a sink, an engineer friend tells me and the lightweight shelf dissipates the energy to the wall.

It might be true. At any rate, there is an audible improvement.
That would depend upon the TT you are using.

Sans that info any replies are clueless.
You guys have me thinking that I may have chosen the wrong material, or perhaps the wrong decoupling method.  Am I better off with a large set of spikes under my acrylic than I am with the Herbies Audio Lab Grungebuster Dots?
I have the Michell Gyrodec which is basically a Gyro on an acrylic platform decoupled with aluminum spikes and aluminum cones as feet on the underside.  Seeing as how Michell chose cones under their acrylic, I'd say that is a good place to start as any.  If it doesn't work out, as least it's not too expensive.
If your table is a non-suspended design: Two shelves of Baltic Birch Plywood separated by a slightly inflated bicycle inner tube, with the table sitting on three roller bearings arranged in an equilateral triangle. The most effective isolation short of a Minus K or similar platform (which cost in excess of $2000).
I purchased a Jean Nantais Lenco Reference Mk III a year ago.  It's a suspension-less design.  Jean recommended a 1/2" slap of marble laminated to a 3/8" think slab of acrylic that is the top layer.  I put Herbie footers under this platform to isolate between my heavy Merrill Stable Table stand.  Works great to my ears. I used maple remnants (beautiful ivory color, all polished and finished off) purchased from a local floor and tile shop - $200, and the acrylic from my local picture framing store - $150.  Cost effective, relatively speaking.
My TT and sits on a 3/8" thick piece of granite.
The granite sits on a 3/4" thick MDF shelf.
Between them is a 1/10" layer of foam
- i.e. the type with holes that you put under a rug to stop it slipping.
- this kills all vibrations in both the MDF and granite
Between the MDF shelf and the steel support for the shelf are 1/10" thick pieces of sorbothane - this kills any vibration from the rack & floor

The TT itself weighs around 25 lbs and has 3 ball bearing feet that sits directly on the granite and between the ball bearing and the TT plinth is another piece of sorbothane

I’ve tried lots of other methods and this is the most vibration free I’ve managed to get it to date.

I use the granite/foam/MDF for all my opponents - I just adjust the size of the granite/foam to a size closer to the size of the component - e.g. my phono stage and DAC sits on a 5/16" thick, 12" x 12" granite tile  which sits on the MDF with foam in between

From this experience I believe the key is to build a shelf from a sandwich of vastly different materials

But - I'm always on the lookout for new ideas :-)
This will only set you back $40 or so give it a try; I use this on my Pro-Ject Xtension 10 and I'm happy with the results and it was easy and cheap.

IKEA Lamplig bamboo cutting board. The board has a grove on one side so I have that side facing down. $14.99 plus shipping.

Between the cutting board and my rack I use 4 RTOM Moongel Dampening Gels. I place the gel pads to the inside edge of the grove noted above; not in the grove but pass the grove. If you live near a Guitar Center or any drum shop they sell the Moongel there. Us drummers have been using this stuff for years. Package with six pads is $7.

This simple cheap tweak works great on all components as well. 
I have a DIY rack from three 3 1/2 in butcher block. The block is manufactured by patching together about 60-80 squares of maple with the grain running in different directions. My thinking, if you can call it that, is that the different orientations would preclude linearity in the wood thus reducing vibrations. I ran threaded brass rod capped with acorn nuts on each end. It sits on Herbies isolation footers. Seems to work great. 

Sand encased in a framed maple box with fixed bottom and floating top is my current method of vibration dampening experimentation. I made a few boxes of varying size settling on one with 6" total depth, subtract 1" for the piece of floating polished granite on which the TT sits upon leaving 5" of kiln dried sand, after allowing adequate time for some settling and an additional layer of sand. Naturally different TT and platform weights along with box depths/ capacities can vary results. Too much weight may over pack the sand causing it to be too uniform lessening its otherwise positive dampening properties. But I think I'm on the right tracks. I may have better results with a lighter piece of plywood or acrylic acting as the floater, and or a deeper sand box to play on.

As with most things having some variety to experiment and fine tune is never a bad Idea. Being accused a typical male as I'm sure many of us have at one point may of been what caused me to reach for the shiniest heaviest material I had in my arsenal knowing full well it may over do it. The lighter ply is still open for evaluation. I think I could live with current arrangement in comparison to some others below. But again I'm a guy so why would I not attempt fixing what isn't necessarily broken.   

I have tried others including steel target shelves, plys, blocks and air suspension made possible with the bike tubes and ingenuity. So far the sand case sounds pretty effective to me. Or the sand hasn't finished settling. After putting forth time and effort if results meet comparable to others the finished sandboxes look great so that brings with it a certain level of contentment. At least for a while.    
If it were me I'd use the roller bearing assembly type Isolators in conjunction with, I.e., on top of the sand filled boxes.  Should be a match made in Heaven given the sandbox is acting primarily in the vertical direction and the roller bearing assemblies act primarily in the horizontal and rotational directions.  
I like the direction williewonka takes but would experiment with the final grounding to the granite. His method does allow for easy checking on the isolation of the granite from vibration--just sprinkle salt or sugar on the granite and play your music. Cheers.. to the eyes...
I use to think granite was a good material, then switched to thick Maple blocks.  They sound much more natural, and still has the dynamics, and speed that granite provides.  So yes Maple is currently still my favorite material.  Obviously what ever your equipment sits on it takes on that character.  I have tried many different materials, carbon fiber, concrete, granite etc.. But wood just sound more natural, less harsh, yet still has good dynamics, speed, but just with a much more natural sound.  I would love to try other woods but the time and money to try them all lol