When I went to the tile store yesterday and picked up a piece of marble to play with Photograv on, the guy kept asking me about what I was doing and as I was explaining, he kept asking "could you do this, could you do that" and he seemed to get excited about it all. After I paid for the tile, he said "hey, wait, this might be good for you" and he started walking me all over the showroom showing me a variety of dark tiles. I explained the whole granite vs. marble thing to him, and he took me to a beautiful piece of very dark travertine tile.
He said it's a limestone or sandstone (shows how well I was listening) based product. He asked if I could use it and I told him I didn't know, but I'd check.
Anyone ever do any photos on darker travertine? I'll pick up a piece next time I'm there if no one's messed with it, and report back.