I have taken some pictures of the Shakabo stone from a recent visit to the Egyptian antiquities collection at the British museum (7/7/12).

The Shabako stone dates from around 700 BC, which is plenty old enough but not as old as the papyrus it was carved to replace.  King Shabako visited the Temple of Ptah and was dismayed to see a truly old scroll being devoured by worms or rats or crocodiles (details sketchy at this point).  He ordered a copy to be carved onto stone.  The stone is deep colored and smooth.  There is a striking pattern carved over the hieroglyphs, a hollow cube in the center with rays moving outwards.  It is a mesmerizing form, with a touch of the void to it.   

Those marks were created later, when the stone was used in a mill to grind wheat.  

When I first saw the stone on an earlier visit, I thought these carvings were intentional.  I saw rays exploding from a central point, and assumed there was  a beautiful meta-commentary on the creation myth inscribed in the stone.  That is not the case, but there is something else going on.  The newer shapes are an echo of who knows how many years human toil, the repetition of a basic human act.  They are the imprint from a stream of individual actors, millers milling.  Unintentionally intentional. 

Some of my pictures show a close up where  the edge of a ray passes over the original glyphs. I can't call it damage or defacement.  The current state is exquisite, the marking created by the millstone.  I get the sense that this is only the latest phase of an evolving form.  It is as much an unfolding in time, unfinished rather than decaying.  The museum is not a collection of relics, but instead it is a workshop for future form.