Bathroom mystery

In the crawl space below the bathroom is a big pile of bathroom remodeling rubble. The stuff was probably swept down there while the built-in bathtub was removed during two different remodelings in the 1950's and 1970s, because most of the original checkerboard tile floor is still intact. In the pile are large pieces of sky blue, black and creamy white tiles. The white tiles are probably of the time of the checkerboard floor; they are stuck to a thick mortar base and, although not intact, could be of subway tile proportions. Perhaps the sky blue and black are from the 50's. Also, there is part of an old wooden door frame which could be the medicine cabinet or a vanity.

Anyhoo, during one hasty trip under the house, from this pile I recovered a broken chunk of fat whitish marble, 3x4-ish with a smooth face, securely stuck to a full thickness of wall plaster. There seems to be a very thin line of limey white between the plaster and marble. What could this be? We live in a very modest house, barely 1000 sq ft. I can't imagine a marble sink back splash or tub surround here. The thickness of the marble makes me think it was once a much bigger piece and I should try to find more of it, but even that can't convince me to go spelunking in the bombed-out ditch under the house. I'd rather go to the dentist. I'd rather x-ray the walls to see where the patched spots are.