Japanese natural stones, (honyama or tennen toishi - literally natural stones) are unique siliceous shales that are found around Kyoto and have been used since the late 12th century for finish sharpening. Named after Honma Touzayemon, the original mountain was named Hon's (shortened form of Honma) Mountain or Hon-Yama and is the location of some of the most famous mines.
The siliceous shales that create the honyama stones of the Tamba formation were formed originally as mud on the sea floor during the Cretaceous period from the skeletons of single celled radiolarians. Radiolarians live their short lives in the upper layers of the ocean and as they die their silicate skeletons drift down to the floor of the ocean to form siliceous mud, a process still going on today. This mud under the correct conditions is lithified and can be scraped up onto the continental crust during subduction of the ocean plate.
Only a small part of the Tamba geological formation around Kyoto contains stones suitable for use as sharpening stones, they need to have just the right amount of compaction, and to be free from mineral inclusions.
The locations of usable stone has been heavily controlled over the centuries as swords and edged weapons were of great strategic importance. As Japan moved from it's more tumultuous warring periods to ones of less conflict, the use of stones for woodworking became the primary driver of stone mining although lots of other sharpening uses were utilized for tennen toishi, including knife sharpening.
The mining of honyama stones around Kyoto has ground nearly to a halt, the majority of mines are either closed or exhausted. The mining of these stones is difficult and potentially dangerous work and as wood construction was replaced by modern architecture the demand for honyama stones fell off steeply. Today demand for honyama stones persists for those doing traditional Japanese carpentry (especially repairing the old temples of Kyoto), those using traditional Japanese shaving razors and culinary knife users.
Honyama sharpen differently than synthetic (jinko-toishi) in that the clumps of flake shaped grit particles from honyama stones can be made to be more or less aggressive with different techniques. One stone can often provide several different finishes depending on how the stones are used; how much water is used and how the slurry is generated. The edges from honyama stones often provide more bite than their synthetic counterparts and cut longer. For many handmade knives the edges provided by honyama stones have a great depth and can bring out many qualities of the steel lost on the more homogenized synthetic stones.
Shoubudani: A ‘Higashi-mono’ (Eastern Mine Stone), Shoubudani is well known for producing a wide range of stones with different hardness and fineness. The Shoubudani mine produces some of the higher quality natural whetstones that are akin to Nakayama mine stones in their beautiful finish.
Aiiwatani: A ‘Higashi-Mono’ (Eastern Mine) whetstone that is close in proximity to Takashima and Wakasa, two mines known for their high density stones. Aiiwatani are great shiage-toishi (fine stones) when you are looking for a refined edge. Typically more mellow in cutting action, but this can lead to a cleaner polish.
Ohira: A ‘Nishi-Mono’ or Western mine that produces some of the highest quality whetstones. This mine is most popular for producing Uchigumori, which is a famous stone type used for sword polishing. For knife users however, we love Ohira Uchigumori to produce a dark jigane and frosty hagane. Ohira mine also produces highly sought after Suita and Tomae stones of various hardness, typically having a fast cutting speed and a toothy yet refined edge.
Atago: A central mine on Mt. Atago that produces large, clean stones of various types like Suita and Tomae. Known for their polishing power and not being too hard, unless stated.