Maybe someone has an idea what this stone is?

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So I bought a Japanese natural stone from a site that deals in second hand tools and they had this stone up for sale. They described it as a relatively soft stone that's also fast from the Kyoto region and in the 4000-5000 range. It was only €70 so I ordered it thinking it would be a good stone for my chisels and plane blades. Well, the description was all wrong, this is a hard stone that leaves a highly polished edge. If I had to guess I'd say it's at least the equivalent of a 10k synthetic stone and I can shave straight of this stone. This the first Japanese natural I have (I'm used to coticules for my razor) and I was wondering if someone might be able to tell me more about it?

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Kiita of some kind maybe from who knows what mine. Love them chisel marks indicates hand finishing for a reason.
MIke

Apparently those chisel marks were used as a form of decoration and they were reserved for high quality stones.