I've never used pieces of Marble as a substrate, but I can tell you that, although it would, no doubt, look really nice, Marble is EXTREMELY expensive! It's very smooth, with basically no texture to it at all, so your beardie most likely would have problems getting a foothold on it. Also, Marble pieces really retain heat, so they may get way to hot for his little feet, because of his heat lamps. Due to all of these concerns, I really wouldn't recommend it!
If you're interested in something that will look really nice, AND be comfortable for him, as well as easy to keep clean, I would recommend a piece of scotchguarded commercial carpeting, like the type that they use in office buildings. It comes in all sorts of colors and patterns, so you can pretty much match any type of decor with it.
If you're at all interested in considering this as a substrate, please let me know, and I'd be glad to post all the particulars on this thread, or send them to you in a Private Message, or an E-Mail to you, if you prefer.
You just can't beat commercial carpeting as a substrate.....it far surpasses anything that I've ever used in comfort for your beardie and ease of care. I've been using the same pieces of commercial carpeting in my three tanks for the last 5 years, and it looks and smells just as nice now as it did the day I put it in there. You just can't beat it as a substrate!
Hope you find this information helpful!