marble problem

 I have a Marble (http://www.lightemprador.com) shower. A few of the Tiles (http://www.g682.net) have small areas where one of the minerals in the tile  has crumbled into tiny shards. When I clean the water off the Tiles (http://www.g682.net) with a window wiper, these tiny, minute bits have fallen out. At first, not knowing where the black bits had come from, I threw them out.