1) нагура имеет вулканичекое происхождение
и представляет собой сцементированную вулканическую пыль
2) нагура является вулканическим туфом
http://ja.wikipedia.org/wiki/%E5%87%9D%E7%81%B0%E5%B2%A9
3) Remember that these 'grades' are not a designation of quality, they only denote the shape and colour of the nagura stone. The 別上 (betsujou) means only that the stone is a predominantly white colour (vs. striped) and in not a uniform square shape (which would be betsu dai jou or 別大上 ). The striped ones are all 特級 and 特級上 (tokkyu and tokkyujou respectively) which are the non white ones, or the so called striped ones which are of the same quality, but are less common.
http://straightrazorplace.com/hones/62721-nagura-types.html
4) The botan is always in katakana i.e. ボタン but there is one that is labeled 八重ボタン or 'yae-botan'.
http://straightrazorplace.com/hones/62721-nagura-types.html
http://www2.odn.ne.jp/mandaraya/5-t-17.htm
5) Для заточки используется не 4, а аж целых 10 видов нагура: http://www2.odn.ne.jp/mandaraya/nagura-youto.htm