its not such a strange idea.
granite has been used in met' labs as the prefered material for surface plates for many years , some 25 years ago I used to work on a bit of kit that was mounted on a granite slab 12ft *6ft8 18" thick at the woolwich arsenal this was flat to within 0.004" across diagonal,
cleaning something like this was with soapy water ,and was a suprisingly robust surface (not easily damaged but could be scratched.
Don't talk to me about Radon. Did I tell the tale of how our Radon reading actually increased with every measure we installed at the NRPB's recommendation? Muppets. :roll: Nothing wrong with a healthy glow-in-the-dark anyway...