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Buying unstamped Nagura is risky. Read JimR's blogpost here.
Eastern Smooth: Stones, Lies and Inkan

Considering how soft these stones are said to be, and how small they are, I wonder how quickly they wear? How long do they last?

Thanks for that picture Andrei, it made for good study to familiarize myself with the different strata kanji. No Koma Nagura?