I'm not sure what Paul has used, but had a lot of tumbled travertine (marble) tiles left from a kitchen remodel we did. I've played around with it some and it seems to work fairly well. It grows coralline fast, and while I've only used a little of it so far, seems to work fine. Since marble is calcium based, I am hoping it will be fine overall. I do soak them in water for a couple weeks, just in case they used any chemicals when they processed them for cleaning or cutting.
You can buy these tiles in boxes of 1 sq foot, which is 9 - 4" tiles for about $5 at either Lowes or HD. They also sell sheets of smaller tiles for borders that work well too, as they are already cut into approximately 1" pieces. If you get these you can pry them off the backing and soak them in water (fresh water) for a week or so and the glue they use on them will soften and you can peel it right off. Otherwise, buy the larger tiles and cut them to size, they cut easily with a tile saw if you happen to have access to one.