Natural Whetstone Quarries of Japan: How Polishing Stones Shape the Art of Sword Finishing

Whetstones: The "Other Material" of Japanese Swords

When discussing Japanese swords, the manufacturing processes of tamahagane, forging, and hardening receive attention, but it is the polisher's (togishi) work that "reveals" the finished blade. The polisher's skill is supported by whetstones—particularly natural whetstones.

Natural whetstones are quarried rocks cut to appropriate sizes, and their polishing characteristics vary greatly depending on origin, layer, and grit (coarseness). Natural whetstone production areas are scattered throughout Japan, but among them, Kameoka City, Kyoto Prefecture (formerly Tanba Province) is known worldwide as the origin of the finest grades.

A polisher uses multiple types of whetstones to finish a single blade, progressing systematically from rough polishing to finishing polish. Since whetstone quality determines the final polish result, whetstone selection is as important to a polisher as technical skill itself.

Natural Whetstone Origins and Types

Kyoto–Kameoka's "Uchigumorido" and "Nakayamado"

The highest grades in Japanese sword polishing are the uchigumorido and nakayamado produced in Kameoka City, Kyoto Prefecture (formerly Tanba Province). These are metamorphic rocks—hard micaceous schists or quartz diorite—with extremely uniform and fine polishing particles and ideal polishing characteristics for steel.

Uchigumorido is the final finishing whetstone used for Japanese sword "finishing polish" (shiagetogi) and is treasured by polishers as the stone that best reveals the workings of the hamon—the patterns of nie, nioi, golden streaks, and other visual characteristics. The production area is concentrated in the Chiyokawa district of Kameoka City, and though some quarrying continues today, high-quality stone becomes increasingly rare year by year.

Nakayamado is often used in combination with "nagurado" and is suited for the intermediate polishing stage (intermediate polish).

Nagurado (Nara—Nagura)

Nagurado, quarried near Tottsukawa Village in Yoshino District, Nara Prefecture, has the characteristic of releasing fine particles (nagura mud) from its surface during polishing. When this nagura mud enters between the whetstone and blade, it functions as an aid-whetstone (jotogi) that creates a uniform polishing surface.

Nagurado is not used alone; rather, it is primarily used by rubbing nagurado with water onto other whetstones to create "todoro" (whetstone slurry) that aids in polishing. High-quality nagurado becomes an indispensable "whetstone conditioning tool" for polishers.

Tanba-whetstone (Nantan City, Kyoto Prefecture)

Tanba-whetstone is a medium-grit whetstone suited for intermediate polishing, quarried near Nantan City adjacent to Kameoka. It has good balance between hardness and fine grit, and is frequently used in the intermediate stage of shaping the blade after rough polishing.

Because its price range is more affordable than uchigumorido, it is also used as a practice stone for polisher apprentices. However, the final result varies greatly depending on how it is combined with the higher-grade uchigumorido, so mastery is required in its use.

Amakusa-whetstone (Amakusa, Kumamoto)

Amakusa-whetstone, produced in the Amakusa region of Kumamoto Prefecture, is a ceramic-type whetstone suited for relatively coarse polishing (equivalent to coarse through medium grit). Beyond Japanese sword polishing, it is widely used for rough polishing of bladed implements in general—knives and carpentry tools. Because it shares composition with ceramic raw materials, it is known as a "dual-production region" for pottery and whetstones.

Comparison with Synthetic Whetstones

In recent years, synthetic whetstones made from compacted aluminum oxide (alundum) and silicon carbide (carborundum) have become widespread. Synthetic whetstones have uniform grit and stable polishing power, with high efficiency in rough and coarse polishing. However, for the final finishing of Japanese swords, the natural variation in fine polishing particles of natural whetstones is said to bring out the "hamon patterns," making natural whetstones indispensable for serious sword polishing.

Current State of Quarrying and the Rarity Problem

The quarrying of natural whetstones relies largely on manual labor by small-scale operators. The Kyoto uchigumorido production area reached its peak in the postwar period, but due to the aging of quarry operators, lack of successors, and rising quarrying costs, production has declined sharply.

A single high-quality uchigumorido stone can cost tens of thousands to hundreds of thousands of yen, and it is not uncommon for polishers to store and collect whetstones as "assets." Whetstones from collections of retired polishers and sword enthusiasts sometimes appear at auction, forming a whetstone market.

As quarrying becomes more difficult, sharing "stone appreciation" expertise between quarry operators and polishers has become an important issue. Even from the same origin, polishing characteristics vary by layer, orientation, and moisture content, so judgments made at the quarry site affect the final product's quality.

Polishers' Whetstone Selection and Storage

"Stone Appreciation"

Polishers use multiple criteria when selecting whetstones. First, "color"—uchigumorido is considered high quality when it has uniform gray-white to pale blue tones; stones with striations or black spots (blackening) risk uneven polishing. Next, "hardness"—stones too hard to fracture have low polishing power and may not wear uniformly. "Surface uniformity"—wetting the cut surface and visually checking particle uniformity.

Whetstone Storage

Natural whetstones risk cracking if they dry too much. Polishers store whetstones either submerged in water or in environments that maintain appropriate humidity. Expensive uchigumorido is kept in dedicated paulownia or wooden boxes, protected from direct sunlight and sudden temperature changes.

"Breaking in" the whetstone is also an important process. Since new whetstones have uneven surfaces, they should be conditioned with light work before being used for serious polishing.

Conclusion: Whetstones—The Unsung Support of Sword Culture

The world of natural whetstone origins, quarrying, and selection is one that sword enthusiasts rarely consciously consider. Yet the brilliance of the hamon that the polisher reveals, the patterns of the ji-tetsu, the shape of the boushi—it is the power of the whetstone that ultimately "shows" all of these.

A single uchigumorido stone extracted from Kameoka's quarries may one day pass into the hands of future polishers a century hence, used to polish the next masterpiece blade. The whetstone is a stone, yet simultaneously a quiet carrier that supports the chain of Japanese sword culture.