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

Whetstones as Another Material for the Japanese Sword

When discussing the Japanese sword, the manufacturing processes of tamahagane, forging, and hardening receive attention, but the task of "displaying" a finished blade belongs to the togishi—the master polisher. And supporting the togishi's craft is the whetstone—particularly the natural whetstone.

Natural whetstones are rocks quarried and cut to appropriate sizes, with polishing characteristics that vary significantly by origin, layer, and grit number. Natural whetstone quarries are scattered throughout Japan, but among them Kameoka City, Kyoto Prefecture (former Tanba province) is internationally recognized as the source of the highest-grade stones.

To finish a single blade, the togishi selects multiple types of whetstones and proceeds through stages of work, from rough polishing to cosmetic finishing. Because whetstone quality determines the final polish, selecting the right whetstone carries importance equal to the togishi's technical skill.

Origins and Types of Natural Whetstones

The following summarizes the names, origins, characteristics, and applications of representative natural whetstones.

Kyoto-Kameoka's "Uchigumo" and "Nakayama" Whetstones

The pinnacle of Japanese sword polishing is represented by the Uchigumo whetstone and Nakayama whetstone from Kameoka City, Kyoto Prefecture (former Tanba province). These are hard crystalline schist (mica schist) or metamorphic quartz diorite, with extremely uniform and fine polishing particles that possess ideal polishing characteristics for steel.

The Uchigumo whetstone is used for the "cosmetic polishing" (finishing polish) of Japanese swords, the final finishing stage, and is treasured by togishi as the stone that best brings out the hamon's appearance—nie, nioi, kinkuji, and other patina details. The quarries are concentrated in the Chiyokawa district of Kameoka City; mining continues today by a limited number of quarrying operations, but high-quality stones are becoming increasingly rare.

The Nakayama whetstone, mined in Umegahata and Kameoka in Kyoto, is positioned as the pinnacle of finishing whetstones when used as a compound stone, employed for the final finishing stage rather than intermediate polishing.

Nagura Whetstone (Nara, Nagura)

The Nagura whetstone, quarried in Aichi Prefecture (Mikawa, Hōrai district of Shinshiro City, etc.), has the characteristic of releasing fine particles (nagura mud) from its surface during polishing. As this nagura mud enters the space between the whetstone and the blade, it functions as an auxiliary stone (josotogi) by forming a uniform polishing surface.

The Nagura whetstone is not used alone; instead, the Nagura stone is rubbed with water over another whetstone to create "todoro" (polishing slurry), which assists in polishing. A high-quality Nagura whetstone becomes an indispensable "regulating stone" for the togishi.

Tanba Whetstone (Nantan City, Kyoto Prefecture)

The Tanba whetstone is a medium-grit whetstone suited for intermediate polishing, quarried in the Nantan area adjacent to Kameoka. It has good balance between hardness and fine abrasive particles, and is widely used in the intermediate stage to shape the blade after rough polishing is complete.

Because the price range is more affordable than the Uchigumo whetstone, it is also used as a practice stone during togishi apprenticeship. However, the final result changes significantly depending on how it is combined with higher-grade Uchigumo whetstones, so mastery of its use requires experience.

Amakusa Whetstone (Amakusa, Kumamoto)

The Amakusa whetstone, produced in the Amakusa region of Kumamoto Prefecture, is a ceramic-type whetstone suited for relatively coarse polishing (rough to medium grit levels). It is widely used for rough polishing of bladed tools generally (kitchen knives, carpentry tools) rather than for Japanese sword polishing. Because it shares composition with ceramic material sources, it is known as a "dual-product region"—both ceramics and whetstones.

Comparison with Synthetic Whetstones

In recent years, synthetic whetstones made from compacted aluminum oxide (corundum) or silicon carbide (carborundum) have become widespread. Synthetic whetstones have uniform grit and stable polishing power, making rough and coarse polishing efficient. However, for the final finishing of Japanese swords, the natural variation in fine polishing particles found in natural whetstones is said to bring out "the patina of the hamon," and natural whetstones are considered essential in serious sword polishing.

Current State of Mining and the Problem of Rarity

Mining of natural whetstones relies largely on small-scale quarrying operations using manual labor. The Uchigumo whetstone quarries of Kyoto reached their peak in the post-war period, but production has declined dramatically due to the aging of quarry operators, shortage of successors, and escalating mining costs.

A single high-quality Uchigumo whetstone can reach tens of thousands to hundreds of thousands of yen, and it is not uncommon for togishi to maintain and collect whetstones as "assets." Whetstones housed by retired togishi or sword enthusiasts sometimes appear at auction, forming a market for these stones.

As mining becomes increasingly difficult, the sharing of "stone appraisal" techniques between quarry operators and togishi has become an important issue. Even within the same origin, polishing characteristics vary by layer, orientation, and moisture content, so decisions made at the quarry site affect the quality of the final product.

Selection and Storage of Whetstones by the Togishi

The "Eye" for Stones

Togishi use multiple criteria when selecting whetstones.

• Color—Uchigumo whetstones are considered high-quality when showing uniform ash-white to pale blue tones; stones with striping or black spots (black striking) may produce uneven polishing
• Hardness—Stones that are too hard and do not crack have low polishing power and may not wear uniformly
• Surface uniformity—The cut surface is examined visually when wetted with water to confirm uniformity of particles

Whetstone Storage

Natural whetstones can crack if dried out excessively. Togishi store whetstones in a water-soaked state (or in environments maintaining appropriate humidity). High-value Uchigumo whetstones are kept in dedicated paulownia or wooden boxes, avoiding direct sunlight and sudden temperature changes.

"Breaking in" the whetstone is also an important process. New whetstones have uneven surfaces, so initially they are used for light work to season the polishing surface before use in serious polishing.

Conclusion: Whetstones as Quiet Supporters of Sword Culture

The world of natural whetstone origins, mining, and selection is an existence rarely consciously noted by sword connoisseurs. Yet the hamon's brilliance that the togishi brings forth through polishing, the patina of the jigane, the shape of the boushi—all of these are ultimately "displayed" by the power of the whetstone.

A single Uchigumo whetstone extracted from the quarries of Kameoka may one day pass into the hands of a future togishi a hundred years hence, to be used in polishing the next masterwork blade. The whetstone is stone, but it is also a quiet bearer supporting the chain of Japanese sword culture.