This is a Hoard stone from the Kyoto Air Raid Shelter. The hand-sawn sides are typical of Meiji and earlier stones. As an Asagi type, with tones of green, and the size suggests it has come from one of the three main  Narutaki mines on the outskirts of Kyoto. 
When the miners uncovered and found this size and grade (quality) of awasedo, the slabs were set aside most often for Buddhist Temple Daiku, those men who built or repaired temples. 
 
There are no inclusions; there is kawa skin and hand chisel work on the backside. The upper left corner in the photo shows that this corner alone is recessed. The stone is a giant at 215 x 75 x 60mm thick (8 1/2 x3x2 1/2 inches thick. No one person could wear out this stone. If it was in a woodworking shop of carpenters, maybe.
Weight is 2476 kg, or 5.424 lb.
 
The hardness of this stone is around 4-Mohs, or 4+ to 4++ on the common scale used outside Japan.
I have honed straight razors, knives, and kanna blades which demand keenness.