This stone appears twice in my book "Japanese Sharpening Stone Heaven". It is a really beautiful and unique stone,  I count on the sides of it to have  12 or so  gas pockets. I will guess that you find some in the middle of the stone, and there is one on the top, I tested it and it did not affect my steel blades.. They do not overwhelm the stone, and as it sits, the stone it cuts steel effortlessly and very finely,.
 
I have tested a whole host of razor steels including Henckels Friodur, Tim Zowada "Timahagane", Brian Brown, handfuls of Japanese steels including kamisori steels my vintage Iwasaki and one Wade & Butcher.
Again, we cannot see the interior of the stone.
 
I rate this stone as a 3+ hardness on the Mohs scale.