A friend of mine knew I wanted a pizza stone, and he picked up a 1" thick slab of travertine for me at Urban Ore (http://urbanore.com/) that fits nicely in my oven.
Functionally it would work great for pizza, but I don't know where this travertine has been or what it's come in contact with. Is it useable? Usable after a special cleaning? Useable with a silicone mat or other protective layer on top? Should I use it as a flagstone instead?