It's actually a closeout travertine tile that we found locally in Atlanta. No one would buy it because of all the dark veining, but that's what we were looking for. We found these 18 x 18 tiles for around $3.50/sf, then had them cut in half.

I suggest going to various stone and tile vendors and ask for the most variegated travertine that they have. Each "lot" will be different, but you might well find what you are looking for with a little digging.

Beautiful.

Thanks Lori!