marble or granite slab under floorstanders?

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

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

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..

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. 

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.