Granite is the answer, go to a tombstone maker or other like ceramic's seller for kitchen or bathroom, you can put lacquer on the edge cut it's more cheaper than polishing and it give the same look and it could last long enough.
I use granite also under my sub Velodyne and all my electronics (Krell amp & SonicFrontiers preamp) are on a piece of granite on spike mount in a damping steel rack made all by myself, very heavy but very stiff and resonance free.
Between the floor and the granite you can use a one inch thick of felt, it's very good for damping and use spike on a wood floor it's not a good idea with several hundred pounds!!!
Also granite looks great :)
I use granite also under my sub Velodyne and all my electronics (Krell amp & SonicFrontiers preamp) are on a piece of granite on spike mount in a damping steel rack made all by myself, very heavy but very stiff and resonance free.
Between the floor and the granite you can use a one inch thick of felt, it's very good for damping and use spike on a wood floor it's not a good idea with several hundred pounds!!!
Also granite looks great :)