Glass blocks! Yes, Maxis included a photo-realistic texture, and yes, it was marked as a floor. This is slightly edited from the image as it comes out of the game files; Maxis' image has two blocks, cloned to make four, but right on top of each other, which would've resulted in very obvious tiling stripes. It will probably also make a better illusion if you have the OFB room divider fences, but in a small enclosed room, it should be fine.
(thanks to CrabofDoom for the above text )