Mosaic Cutter

The philosopher Socrates was said to be a 'mosaic cutter'. This was because of the hemlock in his body; it worked its way up, feet first, until he could no longer report on the progress of its lethal effect. In our Classical past the last piece of a mosaic was inserted loose so that the mosaic cutter could leave via its 'trap door'. We would call it a portal. The author of The Mosaic Cutter discovers a piece of skull bone, dyed black, in with the other pieces he buys online in a sack 'loosely tied at the neck.' Will the 'one previous owner' of it make himself known?