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It's a hard sandstone with good scratch pattern and speed.
If you intend to buy a man made stone, go for a good one. The naniwa superstone 220/1000 is a good stone, but, their chosera 1000 is a lot...

I know that Mr Henk has them, I hope he can tell us what he thinks.
Other options are, of course, Arkansas, and you will not pay a lot for shipping, Wastikivi from Finland, Amakusa and others from...

The site of the company (I think) is this Pierre à aiguiser naturelle des pyrénées.
I contacted them a year ago, they are very friendly and you can ask them for a price catalogue. Their prices are...