Japan — Narutaki district, Ukyo Ward, Kyoto city; stone quarrying documented from at least the Heian period for sword polishing; most premium quarries exhausted by mid-20th century; remaining stones now highly prized
The Narutaki district of Kyoto's northwestern outskirts is the historical source of Japan's most prized natural finishing whetstones — the tennen toishi (natural stones) used by the country's master craftspeople for the final sharpening stages of Japanese knives, woodworking tools, swords (katana), and razors. While natural whetstones are quarried in multiple locations globally, the Kyoto stones from the Narutaki and Ohira areas possess a unique geological character that makes them among the most sought-after sharpening media in the world among professional craftspeople and collectors. The stones are sedimentary rocks formed over millions of years, containing fine-grained silica particles of highly consistent size (typically 3–8 microns) embedded in a clay-silica matrix with natural abrasive properties that differ fundamentally from synthetic stones. The Narutaki region produces several stone varieties within what is collectively called 'Kyoto toishi': the most celebrated include Ohira Uchigumori (used for sword polishing), Suita (fine-grained, hard, for ultra-polished edges), Nakayama (one of the most famous quarries, source of the legendary 'Nakayama Maruka' stones), Koppa (a byproduct-grade natural stone), and Aoto (green stone, used for intermediate sharpening). These natural finishing stones operate through a different mechanism than synthetic stones: they form a 'nagura' slurry during use (either from a separate nagura stone used to raise slurry or from the stone's own matrix) that provides a cutting-and-polishing compound unique to each stone's mineral character. The result — a natural stone apex — has a distinctive 'toothiness' that many craftspeople prefer to the mirror polish of synthetic high-grit stones, claiming it creates a cutting edge that performs better on food materials than a purely polished edge.
Indirectly flavour-defining: a natural-stone-finished yanagiba produces a 'toothy' cutting edge that creates clean cell-division in fish without compression, preserving delicate flavour and texture integrity — the edge quality is inseparable from the sensory result
{"Geological uniqueness: Kyoto toishi formed over millions of years with consistent 3–8 micron silica particle size in clay matrix — cannot be replicated by synthetic stones","Slurry mechanism: natural stones work through self-generated nagura slurry (kasuri) that provides both cutting and polishing action unique to each stone's mineral composition","Stone variety spectrum: Aoto (coarse-intermediate), Uchigumori (intermediate-fine), Suita (fine), Nakayama (ultra-fine) — a complete progression within natural stone tradition","Toothy versus mirror edge: natural stone edges have controlled micro-serration from silica particle variation that some craftspeople prefer for food-cutting applications over synthetic mirror polish","Increasing rarity and value: Nakayama quarry depleted in the late 20th century; surviving Nakayama Maruka stones sell for hundreds to thousands of dollars per stone"}
{"For knife sharpening, use Uchigumori followed by Suita for a sequence that achieves near-mirror polish with some micro-tooth retained — ideal for yanagiba and deba used on delicate fish","Flatten natural stones with a diamond lapping plate before each significant use — natural stones dish over time and an uneven surface undermines the edge geometry","Experienced users 'read' a natural stone's behaviour: different areas of the same stone may have slightly different grit and hardness — learn your stone's character over months of use","Natural stones used with water (as opposed to oil — some traditions use light oil) produce finer slurry more quickly in most Kyoto toishi varieties","At the Kiya cutlery shop in Nihonbashi (Tokyo) and Aritsugu in Kyoto's Nishiki Market, staff can advise on natural toishi selection for specific knife types and skill levels"}
{"Using nagura stone too aggressively — nagura should raise a light slurry, not grind down the sharpening surface; too much nagura friction removes stone material wastefully","Storing natural stones wet — unlike synthetic stones that can be stored in water, many natural toishi develop cracks if stored submerged; store dry and soak briefly before use","Judging natural stones by synthetic standards — natural stones do not sharpen as consistently or quickly as high-quality synthetic alternatives; their value is the character of the resulting edge, not speed","Purchasing without provenance documentation — with natural stones increasingly scarce, forgeries and misrepresented stones are common; reputable dealers provide quarry identification and grade documentation","Using on stainless steel blades — natural finishing stones are primarily suited to high-carbon steel (hagane) blades; modern stainless steel requires more abrasive pressure than natural stones optimally provide"}
The Japanese Kitchen by Hiroko Shimbo; The Art of the Japanese Knife by Hiromitsu Nozaki