marble or granite slab under floorstanders?

Stay away from marble, it causes a ringing characteristic.

Maybe these will help you out 
 
 Gaia Isoacoustics Isolators

 https://www.youtube.com/watch?v=eKCWRCE9Vns

Clearthinker, doesn't matter how much mass you put under a turntable. It will still pick up all kinds of crap. The only adequate solution is a well suspended turntable (SOTA, BASIS, SME, Dohmann) or a MinusK platform. Electron microscopes directly on a concrete floor are a nightmare. This is usually what MinusK platforms are used for. 
As for other equipment? You can theorize that tubes need isolation although I have never seen proof of that. Otherwise it is just another profit making scam perpetrated on audiophiles. The only reasons to put anything under a loudspeaker are to keep the spikes from marring the floor or to get them up to the right height. 

There is a school of thought that says even with a concrete screed floor the Earth's seismic activity will affect loudspeakers with spikes. Similarly footfall on wooden floors will have an impact. There is an app you can download on to a tablet to measure seismic activity by resting the tablet on top of your speakers. The effect manifests itself in the way musical notes decay, but at my age and with my hearing I'm not sure I could hear the difference.

The idea works.  How well is just a question of mass.  More mass, more damping, less ringing, less cabinet/frame movement.

Build listening room in basement.  Dig out, fill in whole floor with new reinforced concrete screed.  Raise speakers each on three metal spikes calibrated to bring them to correct listening height.  Choose your own metal to taste.
There.  Screed is intimate with the earth's mass so you have infinite mass loading.  To all intents and purposes.
I have the same set up for my record player, raised from the concrete floor on large stone and marble slabs.

By the way, if you choose to go stone or marble, don't mess about with 2 or 3 inch thickness.  No way that's enough mass unless you cut them much larger than the speaker footprint - say 5 foot square.

Mass is a wonderful thing.  It allows all your kit to perform to max without external or interdependent vibrations.

I’m sure either works well. Get scraps from a stone supplier and you’ll have to pay for fabrication. Shouldn’t be more than 100 per speaker. 

Better isolation, @Corian  ATH  composite. Granite alternative and much better sonic attributes, Very quiet. An far superior than either marble or granite in terms of "Ring"

It's also easier to work and REPAIR if needed.

Regards..

Google “granite surface plates” which are typically sold at machine shops and are dirt cheap compared to other retailers or kitchen counter shops. 

Go to a counter top dealer...they have scrap pieces to fit your needs Cheaper than a audio dealer....The same product....

Definitely Granit. From a countertop store that hadles stone tops etc. Google it in your geographic area.
You might also want to consider a 3" or 4" thick maple butcher block sold here on Audiogon.
https://www.audiogon.com/stores/bba

Helmut Brinkmann uses 2-3” thick granite under his Model 7 Vandersteen speakers at the Munich audio show and most Of his electronics have an integral Granite base w exception of tables where HRS Designs purpose built bases w constrained layer technology. @ctsooner has a pair of Quattro on granite to great effect I hear..

Don't know which or if either are best under floorstanders. To your second question, I'd suggest checking granite/marble/quartz countertop companies near you, they usually have smaller remnant pieces that they can cut to size