What's the story?

I preached at a church this Sunday that had a cemetery on the grounds.  This is one of the headstones.  The colorful bits on the top and around the edges are small glass flattened marbles.  They are laid on the stone, not attached.  Just loose.

I wonder about these.  Why are they there?  I can see a pattern in their arrangement -- were they placed there one at a time with the pattern developing?  Or were they all added at once?  Is each one a visit?  Is each one a prayer?  Or are they just there to be pretty?

What's the story?