Shironagura Natural Whetstone Pure Mikawa Shironagura whetstones come from the mountain where the original Nagura whetstones were discovered by the Heike clan refugees. The fineness of the grain, especially sought after in sword polishing, is at its best with the fine (koma) grade of Pure Mikawa Shironagura. The mine is currently closed. It was once a popular product found in every shop, but now, with dwindling past stock, it has become difficult to obtain. Due to its rarity, the price has also skyrocketed. Thank you for viewing.