What a great idea. Pieces of granite are often found for free on Craig's list. And if you don't like the look of the plain block you could hunt down decorative block. It comes in a number of patterns.
Bet you could find granite from someone redoing their kitchen or even a box store where the piece is slightly damaged and can't be sold as "perfect".......
I hate losing plants....I know they are "just plants" but when you nurse them along and baby them and get them to grow, bloom, and be pretty and they die....it's like losing a friend.....