Granite instead of an amp stand?

Why not? In my experience, even concrete slabs are effective. This concept also works well as a stable platform for speakers.

The object of a good amp stand is to channel resonant energy away from the amp, but I bet marble would look pretty cool. Give a listen to Stephen Micus/The Music of Stones (ECM 1384). It's oscillating, atmospheric and hypnotic, made on resonating steles cut into metre cubes of black granite and green serpentine.

Well, all I can say is that once I put my Krell fpb 200c on a Zoethecus amp stand, I was stunned with the increase in focus, imaging, etc. I really was amazed!