John,

the blue stone could also have been a Welsh slate, a Thuringian, a
Frankonian or a lesser known metamorphic rock hone... there were many
lesser known quarries providing decent quality hones. I have a yet
unidentified mystery metamorphic hone fragment that came in a tool chest I
purchased a few years ago. The tool chest (nail and rabbet construction)
came from the Belgian Ardennes, made it through both world wars and somehow
survived 50 years in storage before reaching me. The son of the craftsman
was happy to know the tools would be restored and put to use in a workshop.
It was an interesting mix of 19th and early 20th century makers, plus a few
custom tools made by the original owner.

As I keep telling my wife, the rules of the flea market are simple: "if you
put it down, it's gone"

Michael