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http://straightrazorpalace.com/hones/71103-closet-discovery.html

I think the consensus was there are many slates most of which we know under different names. A lot of them are of high quality like...

For razor use you want a stone to be homogeneous without any coarse impurities. If the slate offered looks clean you could give it a try with a (cheap) razor. The grit could be anything between...